Semiconductor device and method for fabricating the same

ABSTRACT

A novel and very useful method for forming a crystal silicon film by introducing a metal element which promotes crystallization of silicon to an amorphous silicon film and for eliminating or reducing the metal element existing within the crystal silicon film thus obtained is provided. The method for fabricating a semiconductor device comprises steps of intentionally introducing the metal element which promotes crystallization of silicon to the amorphous silicon film and crystallizing the amorphous silicon film by a first heat treatment to obtain the crystal silicon film; eliminating or reducing the metal element existing within the crystal silicon film by implementing a second heat treatment within an oxidizing atmosphere; eliminating a thermal oxide film formed in the previous step; and forming another thermal oxide film on the surface of the region from which the thermal oxide film has been eliminated by implementing another thermal oxidation.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device typified by athin film transistor and to a fabrication method thereof. The presentinvention also relates to a semiconductor device using a crystal siliconthin film formed on a substrate such as a glass substrate and quartzsubstrate and to a fabrication method thereof. Further, the presentinvention relates to an insulated gate type semiconductor device such asa thin film transistor and to a fabrication method thereof.

2. Description of Related Art

Hitherto, there has been known a thin film transistor using a siliconfilm, i.e. a technology for forming the thin film transistor by usingthe silicon film formed on a glass substrate or quartz substrate. Theglass substrate or quartz substrate is used as the substrate because thethin film transistor is used for an active matrix liquid crystaldisplay. While a thin film transistor has been formed by using anamorphous silicon film (a-Si) in the past, it is being tried tofabricate the thin film transistor by utilizing a silicon film having acrystallinity (referred to as “crystal silicon film” hereinbelow in thepresent specification as necessary) in order to enhance its performance.

The thin film transistor using the crystal silicon film allows tooperate at a high speed by more than two digits as compared to one usingthe amorphous silicon film. Accordingly, while peripheral drivingcircuits of an active matrix liquid crystal display have been composedof external IC circuits, the crystal silicon film allows them to bebuilt on the glass substrate or quartz substrate similarly to the activematrix circuit. Such structure is very advantageous in miniaturizing thewhole apparatus and in simplifying the fabrication process, thus leadingto the reduction of the fabrication cost.

Hitherto, a crystal silicon film has been obtained by forming anamorphous silicon film by means of plasma CVD or low pressure thermalCVD and then by crystallizing it by implementing a heat treatment or byirradiating laser light. However, it has been the fact that it isdifficult to obtain a required crystallinity across the wide areathrough the heat treatment because it may cause nonuniformity in thecrystallization. Further, although it is possible to obtain the highcrystallinity partly by irradiating laser light, it is difficult toobtain a good annealing effect across the wide area. In this case, theirradiation of the laser light is apt to become unstable under thecondition for obtaining specifically a good crystallinity.

By the way, the inventors et. al. have developed a technology forobtaining the crystal silicon film through a heat treatment at a lowertemperature than that of the prior art by introducing a metal element(e.g. nickel) which promotes the crystallization of silicon to theamorphous silicon film (Japanese Patent Laid-Open Nos. Hei. 6-232059 andHei. 7-321339). These methods allow not only the crystallization speedto be increased and the crystallization to be achieved in a shortertime, but also a high crystallinity to be obtained uniformly across thewide area, thus having a crystallinity which fits for practical use, ascompared to the prior art crystallization of amorphous silicon filmimplemented only by way of heating or by way of the irradiation of laserlight.

However, because the metal element is contained within or on the surfaceof the crystal silicon film, the amount thereof to be introduced has tobe controlled very carefully, thus posing a problem in itsreproducibility and stability (electrical stability of a deviceobtained). Specifically, there is a problem that an elapsed change ofthe characteristics of a semiconductor device to be obtained is large oran OFF value, in case of a thin film transistor, is large, due to theinfluence of the remaining metal element. That is, although the metalelement which promotes the crystallization of silicon plays the valuableand useful role in obtaining the crystal silicon film, its existencebecomes a minus factor which causes various problems after obtaining thecrystal silicon film once.

Then, after conducting a large number of experiments and discussionsfrom various aspects in order to solve the problem in forming thecrystal silicon film by introducing the metal element (e.g. nickel)which promotes the crystallization of silicon to the amorphous siliconfilm and by treating by heat as described above, the inventors et. al.have found that the metal element contained and remaining in the crystalsilicon film may be eliminated or reduced by the specific methoddescribed later, thus reaching to the present invention.

By the way, because an active matrix liquid crystal display is small, islight and is able to display fine motion pictures at high speed, it isbeing expected to become the mainstream of displays of the future.However, because it has a limit that a substrate composing the liquidcrystal display needs to be translucent, its type is limited. A glasssubstrate, a quartz substrate or a plastic substrate may be cited as anexample thereof.

However, among them, the plastic substrate has a problem that it lacksin heat resistance and the quartz substrate has a problem that it isvery expensive and its cost is more than 10 times of the glass substrateespecially when it is widened, thus lacking in cost performance, thoughit can withstand a high temperature of about 1000° C. or 1100° C.Accordingly, the glass substrate is widely used in general from thereasons of heat resistance and economy.

Currently, the performance required for the liquid crystal displays isgetting higher and higher and the performance and characteristicsrequired for a thin film transistor (hereinafter referred to as a TFT asnecessary) used as a switching element of the liquid crystal displays isalso getting higher. Due to that, while the research and development forforming the crystal silicon film having the crystallinity on the glasssubstrate are being actively conducted, the crystal silicon film isformed on the glass substrate by adopting the method of forming theamorphous silicon film and of crystallizing it by treating by heat or byirradiating laser light at the present.

That is, because the heat resistant temperature of the glass substrateis normally about 600° C., though it depends on a type thereof, aprocess which exceeds the heat resistant temperature of the glasssubstrate cannot be adopted in the step for forming the crystal siliconfilm. Therefore, a method for forming the amorphous silicon film bymeans of plasma CVD or low pressure CVD and crystallizing it by heatingat a temperature below that heat resistant temperature has been adoptedin forming the crystal silicon film on the glass substrate. The methodof crystallizing the silicon film by irradiating laser light also allowsthe crystal silicon film having an excellent crystallinity to be formedon the glass substrate and has an advantage that the laser light willnot damage the glass substrate thermally.

However, the crystal silicon film crystallized from the amorphoussilicon film by the above-mentioned technologies has had a large numberof defects caused by dangling bond and the like. Because these defectsare the factor of degrading characteristics of the TFT, it is necessaryto passivate the defects at the interface between an active layer and agate insulating film and the defects within and at the boundary of thecrystal grains of the silicon of the active layer in fabricating the TFTby utilizing such crystal silicon film. The defects at the grainboundary in particular are the greatest factor of scattering charge, butit is very difficult to passivate the defects at the grain boundary.

Meanwhile, it is possible to compensate the defects at the grainboundary of the crystal silicon film by Si in fabricating a TFT on thequartz substrate because it is possible to implement a heat treatment ata high temperature of about 1000° C. or 1100° C. for example. Incontrary to that, it is difficult to implement the heat treatment inhigh temperatures in fabricating the TFT on the glass substrate, so thatthe defects of the grain boundary of the crystal silicon film arepassivated by hydrogen by implementing a hydrogen plasma treatment in anatmosphere of about 300 to 400° C. normally in the final stage of theprocess.

An n-channel type TFT presents a practical field-effect mobility byimplementing the hydrogen plasma treatment. On the other hand, theeffect of the hydrogen plasma treatment is not so remarkable in ap-channel type TFT. It is construed to happen because a level caused bythe defect of the crystal is formed in a relatively shallow domain undera conduction electron zone. Although it is possible to compensate thedefect of the grain boundary of the crystal silicon film by implementingthe hydrogen plasma treatment, an elapsed reliability of the TFT or then-channel type TFT in particular which has been treated by the hydrogenplasma is not stable because the hydrogen compensating the defect is aptto be desorbed. For instance, if the n-channel type TFT is energized for48 hours in an atmosphere of 90° C., its mobility is reduced to a half.

Further, although the quality of the crystal silicon film obtained byirradiating laser light is good, ridges (irregularity) are formed on thesurface of the crystal silicon film if the thickness of the film is lessthan 1000 angstrom. When laser light is irradiated to the silicon film,the silicon film is melt instantly and expands locally. The ridges areformed on the surface of the crystal silicon film to relax internalstress caused by this expansion. A difference of elevation of this ridgeis about ½ to 1 time of the thickness of the film. For instance, whenlaser annealing is implemented after crystallizing an amorphous siliconfilm whose thickness is about 700 angstrom by way of heating, ridges of100 to 300 angstrom in height are formed on the surface thereof.

Because a potential barrier and a trap level caused by the danglingbond, distortion of lattice and the like are formed at the ridges on thesurface of the crystal silicon film in an insulated gate typesemiconductor device, the level of the interface between the activelayer and the gate insulating film becomes high. Further, because thepeak of the ridge is sharp and thus an electric field is apt toconcentrate there, it may become a source of leak current, causingdielectric breakdown in the end. Further, because the ridge on thesurface of the crystal silicon film damages a coating quality of thegate insulating film deposited by way of sputtering or CVD, it degradesthe reliability of insulation by causing defective insulation.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a noveland very useful method for forming a crystal silicon film by introducinga metal element which promotes crystallization of silicon to anamorphous silicon film and for eliminating the metal element or forreducing the concentration of the metal element within the crystalsilicon film thus obtained.

It is another object of the present invention to provide a semiconductordevice having excellent characteristics, and a fabrication methodthereof, fabricated by using a crystal silicon film having a highcrystallinity and obtained by introducing a metal element which promotescrystallization of silicon to an amorphous silicon film and byeliminating the metal element or by reducing the concentration of themetal element in the crystal silicon film.

It is a further object of the present invention to provide asemiconductor device, and a fabrication method thereof, which allows thecharacteristics and reliability of the semiconductor device thusobtained to be enhanced.

It is still another object of the present invention to solve theaforementioned problem by providing a method for fabricating asemiconductor device which allows the defects at the crystal boundary ofthe silicon film crystallized from the amorphous silicon film to bepassivated without using the hydrogen plasma treatment.

It is another object of the present invention to provide a method forfabricating a semiconductor device having a high reliability and highmobility and more particularly to provide a semiconductor device, and afabrication method thereof, which has a gate insulating film composed ofdeposited films, which is formed on a glass substrate and whosereliability and characteristics are enhanced.

While the present invention has objects, beside those described above,which correspond to the structures described below, these will beexplained as necessary complementarily in the description which follows.

In order to solve the aforementioned problems, the present inventionpossesses the following aspects:

-   (1) The present invention provides a method for fabricating a    semiconductor device, comprising steps of intentionally introducing    a metal element which promotes crystallization of silicon to an    amorphous silicon film and crystallizing the amorphous silicon film    by a first heat treatment to obtain a crystal silicon film;    eliminating or reducing the metal element existing within the    crystal silicon film by implementing a second heat treatment within    an oxidizing atmosphere; eliminating a thermal oxide film formed in    the previous step; and forming a thermal oxide film on the surface    of the domain from which the thermal oxide film has been eliminated    by implementing another thermal oxidation.-   (2) The present invention provides a method for fabricating a    semiconductor device, comprising steps of intentionally introducing    a metal element which promotes crystallization of silicon to an    amorphous silicon film and crystallizing the amorphous silicon film    by a first heat treatment to obtain a crystal silicon film;    eliminating or reducing the metal element existing within the    crystal silicon film by implementing a second heat treatment within    an oxidizing atmosphere to form a thermal oxide film on the surface    of the crystal silicon film and by causing the thermal oxide film to    getter the metal element; eliminating the thermal oxide film formed    in the previous step; and forming a thermal oxide film on the    surface of the domain from which the thermal oxide film has been    eliminated by implementing another thermal oxidation.-   (3) The present invention provides a method for fabricating a    semiconductor device, comprising steps of intentionally introducing    a metal element which promotes crystallization of silicon to an    amorphous silicon film and crystallizing the amorphous silicon film    by a first heat treatment to obtain a crystal silicon film;    eliminating or reducing the metal element existing within the    crystal silicon film by implementing a second oxidation heat    treatment within an oxidizing atmosphere; eliminating a thermal    oxide film formed in the steps; forming an active layer of a thin    film transistor by implementing patterning; and forming a thermal    oxide film which composes at least a part of a gate insulating film    on the surface of the active layer by means of thermal oxidation.-   (4) The present invention provides a method for fabricating a    semiconductor device, comprising steps of selectively introducing a    metal element which promotes crystallization of silicon to an    amorphous silicon film; growing crystal by a first heat treatment in    a direction parallel to the film from the domain to which the metal    element has been selectively introduced; forming a thermal oxide    film on the surface of the domain where the crystal has been grown    by implementing a second heat treatment within an oxidizing    atmosphere; eliminating the thermal oxide film; and forming an    active layer of the semiconductor device by using the domain from    which the thermal oxide film has been eliminated.-   (5) The present invention provides a semiconductor device,    characterized in that the semiconductor device has a crystal silicon    film interposed between first and second oxide films; the crystal    silicon film contains a metal element which promotes crystallization    of silicon; and the metal element is distributed in high    concentration near the interfaces with the first and/or second oxide    film within the crystal silicon film.-   (6) The present invention provides a semiconductor device comprising    an underlying layer made from an oxide film; a crystal silicon film    formed on the underlying layer; and a thermal oxide film formed on    the crystal silicon film; wherein the crystal silicon film contains    a metal element which promotes crystallization of silicon; the metal    element which promotes the crystallization of silicon is distributed    in high concentration near the interface with the underlying layer    and/or the thermal oxide film; and the thermal oxide film composes    at least a part of a gate insulating film of a thin film transistor.-   (7) The present invention provides a method for fabricating a    semiconductor device, comprising steps of intentionally introducing    a metal element which promotes crystallization of silicon to an    amorphous silicon film and crystallizing the amorphous silicon film    by a first heat treatment to obtain a crystal silicon film;    eliminating or reducing the metal element existing within the    crystal silicon film by implementing a second heat treatment within    an oxidizing atmosphere containing a halogen element; eliminating a    thermal oxide film formed in the previous step; and forming another    thermal oxide film on the surface of the domain from which the    thermal oxide film has been eliminated by implementing another    thermal oxidation.-   (8) The present invention provides a method for fabricating a    semiconductor device, comprising steps of intentionally introducing    a metal element which promotes crystallization of silicon to an    amorphous silicon film and crystallizing the amorphous silicon film    by a first heat treatment to obtain a crystal silicon film;    eliminating or reducing the metal element existing within the    crystal silicon film by implementing a second heat treatment within    an oxidizing atmosphere containing a halogen element to form a    thermal oxide film on the surface of the crystal silicon film and by    causing the thermal oxide film to getter the metal element;    eliminating the thermal oxide film formed in the previous step; and    forming another thermal oxide film on the surface of the domain from    which the thermal oxide film has been eliminated by implementing    another thermal oxidation.-   (9) The present invention provides a method for fabricating a    semiconductor device, comprising steps of intentionally introducing    a metal element which promotes crystallization of silicon to an    amorphous silicon film and crystallizing the amorphous silicon film    by a first heat treatment to obtain a crystal silicon film;    eliminating or reducing the metal element existing within the    crystal silicon film by implementing a second heat treatment within    an oxidizing atmosphere containing a halogen element; eliminating a    thermal oxide film formed in the previous step; forming an active    layer of a thin film transistor by implementing patterning; and    forming another thermal oxide film which composes at least a part of    a gate insulating film on the surface of the active layer by means    of thermal oxidation.-   (10) The present invention provides a method for fabricating a    semiconductor device, comprising steps of selectively introducing a    metal element which promotes crystallization of silicon to an    amorphous silicon film; growing crystal by a first heat treatment in    a direction parallel to the film from the domain to which the metal    element has been selectively introduced; forming a thermal oxide    film on the surface of the domain where the crystal has been grown    by implementing a second heat treatment within an oxidizing    atmosphere containing a halogen element; eliminating the thermal    oxide film; and forming an active layer of the semiconductor device    by using the domain from which the thermal oxide film has been    eliminated.-   (11) The present invention provides a semiconductor device,    characterized in that the semiconductor device has a crystal silicon    film interposed between a first and second oxide films; the crystal    silicon film contains hydrogen and a halogen element as well as a    metal element which promotes crystallization of silicon; and the    metal element is distributed in high concentration near the    interfaces with the first and/or second oxide film within the    crystal silicon film.-   (12) The present invention provides a semiconductor device,    comprising an underlying layer made from an oxide film; a crystal    silicon film formed on the underlying layer; and a thermal oxide    film formed on the crystal silicon film; wherein the crystal silicon    film contains a metal element which promotes crystallization of    silicon, hydrogen and a halogen element; the metal element which    promotes the crystallization of silicon is distributed in high    concentration near the interface with the underlying layer and/or    the thermal oxide film; the halogen element is distributed in high    concentration near the interface with the underlying layer and/or    the thermal oxide film; and the thermal oxide film composes at least    part of a gate insulating film of a thin film transistor.-   (13) The present invention provides a method for fabricating a    semiconductor device, comprising steps of intentionally introducing    a metal element which promotes crystallization of silicon to an    amorphous silicon film and crystallizing the amorphous silicon film    by a first heat treatment to obtain a crystal silicon film;    irradiating laser light or intense light to the crystal silicon    film; eliminating or reducing the metal element existing within the    crystal silicon film by implementing a second heat treatment within    an oxidizing atmosphere containing a halogen element; eliminating a    thermal oxide film formed in the previous step; and forming another    thermal oxide film on the surface of the domain from which the    thermal oxide film has been eliminated by implementing another    thermal oxidation.-   (14) The present invention provides a method for fabricating a    semiconductor device, comprising steps of intentionally introducing    a metal element which promotes crystallization of silicon to an    amorphous silicon film and crystallizing the amorphous silicon film    by a first heat treatment to obtain a crystal silicon film;    irradiating laser light or intense light to the crystal silicon film    to diffuse the metal element, existing within the crystal silicon    film, in the crystal silicon film; implementing a second heat    treatment within an oxidizing atmosphere containing a halogen    element to cause the metal element existing within the crystal    silicon film to be gettered to a thermal oxide film to be formed;    eliminating the thermal oxide film formed in the previous step; and    forming another thermal oxide film on the surface of the domain from    which the thermal oxide film has been eliminated by implementing    another thermal oxidation.-   (15) The present invention provides a method for fabricating a    semiconductor device, comprising steps of intentionally and    selectively introducing a metal element which promotes    crystallization of silicon to an amorphous silicon film;    implementing a first heat treatment to the amorphous silicon film to    grow crystal in a direction parallel to the film from a domain of    the amorphous silicon film into which the metal element has been    intentionally and selectively introduced; irradiating laser light or    intense light to diffuse the metal element existing within the    domain where the crystal has grown; implementing a second heat    treatment within an oxidizing atmosphere containing a halogen    element to cause the metal element existing within the domain where    the crystal has grown to be gettered to a thermal oxide film to be    formed; eliminating the thermal oxide film formed in the previous    step; and forming another thermal oxide film on the surface of the    domain from which the thermal oxide film has been eliminated by    implementing another thermal oxidation.-   (16) The present invention provides a method for fabricating a    semiconductor device, comprising steps of intentionally introducing    a metal element which promotes crystallization of silicon to an    amorphous silicon film and crystallizing the amorphous silicon film    by a first heat treatment to obtain a crystal silicon film; forming    an active layer of the semiconductor device by patterning the    crystal silicon film; irradiating laser light or intense light to    the active layer; implementing a second heat treatment within an    oxidizing atmosphere containing a halogen element to eliminate or    reduce the metal element existing within the active layer;    eliminating a thermal oxide film formed in the previous step; and    forming another thermal oxide film on the surface of the active    layer by implementing another thermal oxidation.-   (17) The present invention provides a method for fabricating a    semiconductor device, comprising steps of intentionally introducing    a metal element which promotes crystallization of silicon to an    amorphous silicon film and crystallizing the amorphous silicon film    by a first heat treatment to obtain a crystal silicon film; forming    an active layer of the semiconductor device by patterning the    crystal silicon film; irradiating laser light or intense light to    the active layer; implementing a second heat treatment within an    oxidizing atmosphere containing a halogen element to eliminate or    reduce the metal element existing within the active layer;    eliminating a thermal oxide film formed in the previous step; and    forming another thermal oxide film on the surface of the active    layer by implementing another thermal oxidation, wherein the active    layer has an inclined shape in which an angle formed between a side    face and an underlying face is 20° to 50°.-   (18) The present invention provides a method for fabricating a    semiconductor device, comprising steps of intentionally introducing    a metal element which promotes crystallization of silicon to an    amorphous silicon film and crystallizing the amorphous silicon film    by a first heat treatment to obtain a crystal silicon film;    irradiating laser light or intense light to the crystal silicon    film; eliminating or reducing the metal element existing within the    crystal silicon film by implementing a second heat treatment within    an oxidizing atmosphere; eliminating a thermal oxide film formed in    the previous step; and forming another thermal oxide film on the    surface of the domain from which the thermal oxide film has been    eliminated by implementing another thermal oxidation.-   (19) The present invention provides a method for fabricating a    semiconductor device, comprising steps of intentionally introducing    a metal element which promotes crystallization of silicon to an    amorphous silicon film and crystallizing the amorphous silicon film    by a first heat treatment to obtain a crystal silicon film;    irradiating laser light or intense light to the crystal silicon film    to diffuse the metal element, existing within the crystal silicon    film, in the crystal silicon film; implementing a second heat    treatment within an oxidizing atmosphere to cause the metal element    existing within the crystal silicon film to be gettered to a thermal    oxide film to be formed; eliminating the thermal oxide film formed    in the previous step; and forming another thermal oxide film on the    surface of the domain from which the thermal oxide film has been    eliminated by implementing another thermal oxidation.-   (20) The present invention provides a method for fabricating a    semiconductor device, comprising steps of intentionally and    selectively introducing a metal element which promotes    crystallization of silicon to an amorphous silicon film;    implementing a first heat treatment to the amorphous silicon film to    grow crystal in a direction parallel to the film from a domain of    the amorphous silicon film into which the metal element has been    intentionally and selectively introduced; irradiating laser light or    intense light to diffuse the metal element existing within the    domain where the crystal has grown; implementing a second heat    treatment within an oxidizing atmosphere to cause the metal element    existing within the domain where the crystal has grown to be    gettered to a thermal oxide film to be formed; eliminating the    thermal oxide film formed in the previous step; and forming another    thermal oxide film on the surface of the domain from which the    thermal oxide film has been eliminated by implementing another    thermal oxidation.-   (21) The present invention provides a method for fabricating a    semiconductor device, comprising steps of intentionally introducing    a metal element which promotes crystallization of silicon to an    amorphous silicon film and crystallizing the amorphous silicon film    by a first heat treatment to obtain a crystal silicon film; forming    an active layer of the semiconductor device by patterning the    crystal silicon film; irradiating laser light or intense light to    the active layer; implementing a second heat treatment within an    oxidizing atmosphere to eliminate or reduce the metal element    existing within the active layer; eliminating a thermal oxide film    formed in the previous step; and forming another thermal oxide film    on the surface of the active layer by implementing another thermal    oxidation.-   (22) The present invention provides a method for fabricating a    semiconductor device, comprising steps of intentionally introducing    a metal element which promotes crystallization of silicon to an    amorphous silicon film and crystallizing the amorphous silicon film    by a first heat treatment to obtain a crystal silicon film; forming    an active layer of the semiconductor device by patterning the    crystal silicon film; irradiating laser light or intense light to    the active layer; implementing a second heat treatment within an    oxidizing atmosphere to eliminate or reduce the metal element    existing within the active layer; eliminating a thermal oxide film    formed in the previous step; and forming another thermal oxide film    on the surface of the active layer by implementing another thermal    oxidation, wherein the active layer has an inclined shape in which    an angle formed between a side face and an underlying face is 20° to    50°.-   (23) The present invention provides a method for fabricating a    semiconductor device, comprising steps of forming an amorphous    silicon film on a substrate having an insulating surface;    intentionally introducing a metal element which promotes    crystallization of silicon to the amorphous silicon film; obtaining    a crystal silicon film by crystallizing the amorphous silicon film    by a first heat treatment in the temperature range of 750° C. to    1100° C.; forming an active layer of the semiconductor device by    patterning the crystal silicon film; eliminating or reducing the    metal element existing within the crystal silicon film by    implementing a second heat treatment within an oxidizing atmosphere    containing a halogen element; eliminating a thermal oxide film    formed in the previous step; and forming another thermal oxide film    after eliminating the thermal oxide film by implementing another    thermal oxidation; wherein a temperature of the second heat    treatment is higher than that of the first heat treatment.-   (24) The present invention provides a method for fabricating a    semiconductor device, comprising steps of forming an amorphous    silicon film on a substrate having an insulating surface;    intentionally introducing a metal element which promotes    crystallization of silicon to the amorphous silicon film; obtaining    a crystal silicon film by crystallizing the amorphous silicon film    by a first heat treatment in the temperature range of 750° C. to    1100° C.; forming an active layer of the semiconductor device by    patterning the crystal silicon film; implementing a second hear    treatment within an oxidizing atmosphere containing a halogen    element to cause the metal element existing within the crystal    silicon film to be gettered to a thermal oxide film to be formed;    eliminating the thermal oxide film formed in the previous step; and    forming another thermal oxide film after eliminating the thermal    oxide film by implementing another thermal oxidation; wherein a    temperature of the second heat treatment is higher than that of the    first heat treatment.-   (25) The present invention provides a method for fabricating a    semiconductor device, comprising steps of forming an amorphous    silicon film on a substrate having an insulating surface;    intentionally and selectively introducing a metal element which    promotes crystallization of silicon to the amorphous silicon film;    growing crystal in a direction parallel to the film from a domain of    the amorphous silicon film into which the metal element has been    intentionally and selectively introduced by a first heat treatment    in the temperature range of 750° C. to 1100° C.; forming an active    layer of the semiconductor device by using the domain in which the    crystal has been grown in the direction parallel to the film by    patterning; implementing a second heat treatment within an oxidizing    atmosphere containing a halogen element to cause the metal element    existing within the active layer to be gettered to a thermal oxide    film to be formed; eliminating the thermal oxide film formed in the    previous step; and forming another thermal oxide film after    eliminating the thermal oxide film by implementing another thermal    oxidation; wherein a temperature of the second heat treatment is    higher than that of the first heat treatment.-   (26) The present invention provides a method for fabricating a    semiconductor device, comprising steps of forming an amorphous    silicon film; holding a metal element which promotes crystallization    of silicon in contact on the surface of the amorphous silicon film;    crystallizing the amorphous silicon film by a first heat treatment    to obtain a crystal silicon film; forming a thermal oxide film on    the surface of the crystal silicon film by implementing a second    heat treatment in the temperature range of 500° C. to 700° C. within    an atmosphere containing oxygen, hydrogen and fluorine; and    eliminating the thermal oxide film.-   (27) The present invention provides a method for fabricating a    semiconductor device, comprising steps of forming an amorphous    silicon film; holding a metal element which promotes crystallization    of silicon in contact on the surface of the amorphous silicon film;    crystallizing the amorphous silicon film by a first heat treatment    to obtain a crystal silicon film; forming a thermal oxide film on    the surface of the crystal silicon film by implementing a second    heat treatment in the temperature range of 500° C. to 700° C. within    an atmosphere containing oxygen, hydrogen, fluorine and chlorine;    and eliminating the thermal oxide film.-   (28) The present invention provides a method for fabricating a    semiconductor device, comprising steps of forming an amorphous    silicon film; holding a metal element which promotes crystallization    of silicon in contact on the surface of the amorphous silicon film;    obtaining a crystal silicon film by crystallizing the amorphous    silicon film by a heat treatment; forming a wet oxide film on the    surface of the crystal silicon film within an atmosphere containing    fluorine/chlorine; and eliminating the oxide film.-   (29) The present invention provides a semiconductor device having a    silicon film having a crystallinity, characterized in that the    silicon film contains a metal element which promotes crystallization    of silicon in concentration of 1×10¹⁶ cm⁻³ to 5×10¹⁸ cm⁻³, fluorine    atoms in concentration of 1×10¹⁵ cm⁻³ to 1×10²⁰ cm⁻³, and hydrogen    atoms in concentration of 1×10¹⁷ cm⁻³ to 1×10²¹ cm⁻³. It is noted    that the unit of concentration “ . . . cm⁻³” means the number of    atoms (atoms/cm³) per 1 cc and the same applies throughout the    present specification.-   (30) The present invention provides a method for fabricating a    semiconductor device, comprising steps of forming an amorphous    silicon film; crystallizing the amorphous silicon film to form a    crystal silicon film; growing a thermal oxide film on the surface of    the crystal silicon film by heating in an oxidizing atmosphere to    which fluorine compound gas is added; eliminating the thermal oxide    film on the surface of the crystal silicon film; and depositing an    insulating film on the surface of the crystal silicon film.-   (31) The present invention provides a method for fabricating a    semiconductor device, comprising steps of forming an amorphous    silicon film; irradiating laser light to crystallize the amorphous    silicon film to form a crystal silicon film; growing a thermal oxide    film on the surface of the crystal silicon film by heating in an    oxidizing atmosphere to which fluorine compound gas is added;    eliminating the thermal oxide film on the surface of the crystal    silicon film; and depositing an insulating film on the surface of    the crystal silicon film.-   (32) The present invention provides a method for fabricating a    semiconductor device in fabricating a thin film transistor on a    substrate having an insulating surface, comprising steps of forming    an amorphous silicon film; crystallizing the amorphous silicon film    to form a crystal silicon film; growing a thermal oxide film on the    surface of the crystal silicon film by heating in an oxidizing    atmosphere to which fluorine compound gas is added; eliminating the    thermal oxide film on the surface of the crystal silicon film;    forming an active layer of the thin film transistor by shaping the    crystal silicon film; depositing an insulating film on the surface    of the active layer to form a gate insulating film at least on the    surface of a channel region; forming a gate electrode on the surface    of the gate insulating film; and forming a source and a drain in a    manner of self-alignment by injecting impurity ions which give a    conductive type to the active layer by using the gate electrode as a    mask.-   (33) The present invention provides a method for fabricating a    semiconductor device in fabricating a thin film transistor on a    substrate having an insulating surface, comprising steps of forming    an amorphous silicon film; forming a crystal silicon film by    crystallizing the amorphous silicon film; irradiating laser light to    the crystal silicon film; growing a thermal oxide film on the    surface of the crystal silicon film by heating in an oxidizing    atmosphere to which fluorine compound gas is added; eliminating the    thermal oxide film on the surface of the crystal silicon film;    forming an active layer of the thin film transistor by shaping the    crystal silicon film; depositing an insulating film on the surface    of the active layer to form a gate insulating film at least on the    surface of a channel region; forming a gate electrode on the surface    of the gate insulating film; and forming a source and a drain in a    manner of self-alignment by injecting impurity ions which give a    conductive type to the active layer by using the gate electrode as a    mask.

According to one typical aspect of the present invention, a metalelement which promotes crystallization of silicon is introduced to thesurface of an amorphous silicon film formed in advance to form a crystalsilicon film. Next, a thermal oxide film is formed on the surface of thecrystal silicon film to cause the metal element to move or to begettered to the thermal oxide film to reduce the concentration of themetal element or to eliminate the metal element within the crystalsilicon film.

The amorphous silicon film may be formed by means of normal methods suchas plasma CVD. The amorphous silicon film is formed on a surface ofadequate solid body or on a substrate when it is used in constructing asemiconductor device. For the substrate, a ceramic substrate or thelike, beside a glass substrate and a quartz substrate, may be used.While the amorphous silicon film is formed also on a film such as asilicon oxide film formed on the surface of such substrate, thesubstrate which is referred in the present specification means toinclude such aspects.

Next, the metal element which promotes the crystallization of silicon isintroduced to the surface of the amorphous silicon film formed inadvance as described above. As the metal element which promotes thecrystallization of silicon, one or a plurality of types of metalelements selected from iron (Fe), nickel (Ni), cobalt (Co), ruthenium(Ru), rhodium (Rh), paradium (Pd), osnium (Os), iridium (Ir), platinum(Pt), copper (Cu), and gold (Au) is used. These metal elements are usedas the metal elements which promote the crystallization of silicon inany of the inventions described in the present specification and arereferred in the present specification as “metal elements which promotethe crystallization of silicon typified by nickel” as necessary.

While these metal elements may be introduced: 1) on the whole surface ofthe amorphous silicon film, 2) at end portions of the amorphous siliconfilm (if the face of the amorphous silicon film is rectangular, at theend of one side, ends of two sides, ends of three sides or ends of foursides: if the face of the amorphous silicon film is circular, at itsperipheral portion), 3) to the center of the face of the amorphoussilicon film, 4) in dots (that is, in dots leaving predetermined spacestherebetween on the surface of the amorphous silicon film) and the likeand there is no specific limitation, it is preferred to introduce on thewhole surface or to the end portions of the amorphous silicon film.Further, although it is possible to adopt an aspect of introducing themetal element on the back face of the amorphous silicon film, it ispreferable to apply it on the front surface from the aspect offabrication of the semiconductor device.

Further, there is no specific limit on the method how to introduce thosemetal elements to the amorphous silicon film so long as it is a methodwhich allows the metal elements to be introduced to the surface or theinside of the amorphous silicon film, and such methods as sputtering,CVD, plasma treatment (including plasma CVD), adsorption and a method ofapplying solution of metallic salt may be used for example. Among them,the method of using solution is useful from the aspects that it issimple and that the concentration of the metal element may be readilyadjusted. Various salts may be used for the metallic salt and organicsolvents such as alcoholic, aldehyde, ether solvents or a mixed solventof water and organic solvent may be used beside water as the solvent.Further, the solution needs not be what such metallic salt is dissolvedcompletely and may be what part or whole of the metallic salt exists insuspended state.

Any type of the metallic salt can be used regardless whether it isorganic salt or non-organic salt so long as it is salt which can existsas the solution or suspended solution as described above. For example,such ferrous salt as ferrous bromide, ferric bromide, ferric acetate,ferrous chloride, ferric chloride, ferric fluoride chloride, ferricnitrate, ferrous phosphate, ferric phosphate and the like and suchcobalt salt as cobalt bromide, cobalt acetate, cobalt chloride, cobaltfluoride, cobalt nitrate and the like may be used.

Further, such nickel salt as nickel bromide, nickel acetate, nickeloxalate, nickel carbonate, nickel chloride, nickel iodide, nickelnitrate, nickel sulfate, nickel formate, nickel oxide, nickel hydroxide,nickel acetylacetate, nickel 4-cyclohexylbutyrate, nickel etylhexanoicacid and the like may be used. Ruthenium chloride may be cited as anexample of ruthenium salt, rhodium chloride as an example of rhodiumsalt, paradium chloride as an example of paradium salt, osnium chlorideas an example of osmium salt, iridium trichloride or iridiumtetrachloride as examples of iridium salt, platinic chloride as anexample of platinum salt, cupric acetate, cupric chloride and cupricnitrate as examples of copper salt, and gold trichloride and goldchloride as gold salt.

After thus introducing the metal element to the amorphous silicon film,the crystal silicon film is formed by using the metal element. Whilethis crystallization may be carried out by implementing a heat treatment(Solid Phase Crystallization) or by irradiating laser light or intenselight such as ultraviolet ray or infrared ray, it is preferable to usethe heat treatment. While this solid phase crystallization proceeds evenin an atmosphere containing hydrogen or oxygen, preferably an inactiveatmosphere such as nitrogen or argon is used. It is noted that this heattreatment or the heat treatment temperature will be referred to as the“first heat treatment” or “temperature of the first heat treatment” asnecessary throughout the present specification.

The first heat treatment may be carried out in the temperature range of400 to 1100° C. or preferably about 550 to 1050° C. Although thecrystallization proceeds even at a temperature of about 400° C., thecrystallization speed is slow and it takes a long time in this case.Accordingly, the temperature is preferable to be above 550° C. or morepreferably above 700° C. The higher heat treatment temperature allows abetter quality crystal to be obtained and the crystallization speed tobe increased.

While the first heating temperature is limited to about 600 to 650° C.from the aspect of distortion point when a glass substrate whosedistortion point is 667° C. for example is used as the substrate, it isneedless to say that the temperature may be increased further if a glasssubstrate having a high heat resistance is used. While a temperature ofabout 1100° C. may be applied when the substrate is a quartz substrate,it is preferred to be less than about 1050° C. Further, the irradiationof laser light or intense light may be carried out after the heattreatment.

Then, the thermal oxide film is formed on the surface of the crystalsilicon film. Thereby, the concentration of the metal element within thecrystal silicon film may be reduced or the metal element may beeliminated by causing the metal element to move into or to be getteredto the thermal oxide film according to the present invention. While anoxidizing atmosphere is used in forming the thermal oxide film, it ispreferably 1) an oxygen atmosphere, 2) an atmosphere containing oxygen,3) an atmosphere containing a compound which releases oxygen at thetemperature in forming the thermal oxide film, or 4) an atmospherecontaining oxygen in 1), 2) and 3) above and halogen.

The thermal oxide film may be formed in the same temperature range withthat of the solid phase crystallization described above, i.e. in therange of about 400 to 1100° C., or preferably about 700 to 1050° C.While this temperature may be about the same with that applied to thesolid phase crystallization (temperature of the first heat treatment),it is preferable to be higher than that applied to the solid phasecrystallization. Thereby, the thermal oxide film may be formed and thesolid phase crystallization may be advanced further as compared to thecase when the same temperature with that of the first heat treatment isapplied.

While the thermal oxide film is thus formed on the surface of thecrystal silicon film, the effect of oxygen or oxygen and halogen withinthe oxidizing atmosphere causes the metal element to be gettered to thethermal oxide film and the concentration of the metal element within thecrystal silicon film is reduced or the metal element is eliminated. Itis noted that the heat treatment for forming the thermal oxide film andits temperature are referred to “second heat treatment” and “temperatureof the second heat treatment” as necessary throughout the presentspecification.

Next, the thermal oxide film which has gettered the metal element iseliminated. Although there is no limit on the method for eliminating thethermal oxide film so long as it is a method which can eliminate thethermal oxide film, it may be carried out by using a hydrofluoric acidtype etchant such as buffer hydrofluoric acid. Thus, the crystal siliconfilm having a high crystallinity and from which the metal element hasbeen eliminated or the concentration of the metal element has beenreduced can be obtained. This crystal silicon film has excellentcharacteristics as an element (e.g. active layer) within a semiconductordevice.

FIGS. 1 through 4 show microphotographs of several examples of thecrystal silicon film. Among the figures, FIG. 1 shows a case in whichnickel element has been applied to one end of a rectangular amorphoussilicon film to crystallize it and FIG. 2 shows a case in which nickelelement has been applied to the whole surface of an amorphous siliconfilm to crystallize it. As it is apparent from FIG. 1, the crystal hasgrown from one end to the other end in parallel or almost in parallel.In the example in FIG. 2 in which nickel element has been applied to thewhole surface of the amorphous silicon film to grow crystal, star-likelight and shades can be seen and it can be seen that crystals have grownradially centering on a number of points.

FIGS. 3 and 4 are photographs taken by a transmission type electronmicroscope. The crystal silicon film shown-in the photographs is whathas been obtained approximately through the process of (A) through (G)in FIGS. 5A through 5G which show the process diagrammatically (theprocess similar to those in embodiments described later).

-   (A) A quartz substrate having a fully smooth surface is cleaned and    an amorphous silicon film is formed thereon in a thickness of 500    angstrom by means of low pressure thermal CVD (LPCVD).-   (B) Next, a silicon oxide film is formed in a thickness of 700    angstrom by means of CVD using TEOS (tetraethoxisilane) and an    opening is formed by patterning it. The amorphous silicon is exposed    at the bottom of the opening.-   (C) Nickel acetate solution containing nickel in concentration    (weight conversion) of 100 ppm is applied as shown in FIG. 5 c by a    spin-coater.-   (D) A heat treatment is implemented within a nitrogen atmosphere at    600° C. for eight hours in the state while adhering the nickel    acetate solution.-   (E) A mask of the silicon oxide film is removed to obtain a crystal    silicon film having a region where crystal has grown laterally.-   (F) A heat treatment is implemented within an oxygen atmosphere    (atmospheric pressure) containing 3 volume % of HCl at 950° C. for    20 minutes. As a result, an oxide film of 200 angstrom is formed and    the thickness of the silicon film is reduced to 400 angstrom. It is    noted that while the reason why the thickness of the crystal silicon    film is reduced is unknown in detail and we would have to wait for    the study of the future, it is assumed to have happened because    silicon in non-crystallized state or not crystallized completely is    consumed in the formation of the thermal oxide film.-   (G) The oxide film formed in step (F) is eliminated by using buffer    hydrofluoric acid.

As it is apparent from FIGS. 3 and 4, the crystal in the crystal siliconfilm has grown 1) such that a structure of crystal lattices liescontinuously in a row, 2) so as to be thin cylindrical crystal or thinflat cylindrical crystal, and 3) so as to be a plurality of thincylindrical crystals or thin flat cylindrical crystals in parallel oralmost in parallel leaving a space therebetween. Further, seeing thephotograph of FIG. 4, it can be seen that a cylindrical crystal of 0.15μm in width extends diagonally from the lower right corner to the upperleft corner and that there is a clear boundary (grain boundary) at theedges of the both ends.

FIGS. 7 a and 7 b show the form of crystal growth in the crystal siliconfilm obtained by the present invention and assumed from the resultobserved from a number of photographs of the electron microscopetypified by FIGS. 1 through 4. FIG. 7 a shows one exemplary case whenthe crystal has been grown by introducing the metal element whichpromotes the crystallization of silicon into one end of the surface ofthe amorphous silicon film. In this case, the crystals of silicon growlinearly from the region where the metal has been added in parallel oralmost in parallel.

FIG. 7 b shows a case when the crystal has been grown by applying themetal element which promotes the crystallization of silicon on the wholesurface of the amorphous silicon film. In this case, the crystals ofsilicon grow radially centering from a numerous points on the wholesurface of the amorphous silicon film. Seeing from the mutualrelationship of position of adjacent radial crystal cylinders whichextend centering from respective points, each crystal grows linearly andin parallel or almost in parallel.

By the way, it is effective to shorten a channel length in order toincrease an operating speed of a TFT for example (while the same appliesto a MOS type transistor in general, this point will be describedcentering on TFT here) However, if the channel length is shortened below1 μm for example, a trouble called a short-channel effect is broughtabout. In concrete, problems such as the degradation of sub-thresholdcharacteristic and the decrease of threshold value occur.

Here, the sub-threshold characteristic (referred also as S value) meansa build-up characteristic when a switch of the TFT is turned on as showndiagrammatically in FIG. 8. In concrete, if the build-up is sharp, thesub-threshold characteristic is good and the TFT may be operated at highspeed. On the other hand, a TFT having a bad sub-thresholdcharacteristic has a build-up curve whose inclination is small (i.e. thecurve is lying) and it is not suited to high-speed operation.

The degradation of the sub-threshold characteristic in the short-channeleffect may be explained from the present technological knowledge(=present technological knowledge or prior art theory) as follows.Firstly, what the channel is shortened means that a distance between asource region and a drain region is shortened. Generally, a channel isintrinsic (I type semiconductor) and a source/drain region is N- orP-type semiconductor. If an intrinsic semiconductor contacts with anN-type semiconductor for example, the quality of the N-typesemiconductor as the semiconductor exerts influence to the inside of theintrinsic semiconductor.

In the case of the TFT, the above-mentioned influence is exerted to theinside of the channel. That is, the influence of N-type or P-type isexerted from the source region or drain region to the inside of thechannel. The degree of this influence, i.e. a range in which theinfluence is exerted, does not change even if the channel is shortened.

If the channel length is shortened further, the influence exerted fromthe source/drain region to the channel with respect to the size of thechannel length becomes significant. In an extreme case, the range of theinfluence exerted from the source/drain region to the inside of thechannel may become longer than the channel length. In such a state, atrouble occurs in the operation of the TFT (the same applies also to MOStype transistors) that the change of conductive type of the channel iscontrolled by the application of electric field from a gate electrodeand electric conductivity between the source and the drain changes, thusdegrading the sub-threshold characteristic as a result.

A TFT using the crystal silicon film obtained by the present inventionhas a channel length of less than about 1 μm. Accordingly, it ispresumed that the short-channel effect appears naturally from thetechnological knowledge as described above.

However, it has been found that in the crystal of the crystal siliconfilm obtained by the present invention, i.e. the crystal which havegrown 1) such that a structure of crystal lattices lies continuously ina row, 2) so as to be thin cylindrical crystal or thin flat cylindricalcrystal, and 3) so as to be a plurality of thin cylindrical crystals orthin flat cylindrical crystals in parallel or almost in parallel leavinga space therebetween, not only no short-channel effect is seen, but alsoa very good sub-threshold characteristic which cannot be explained bythe prior art technological knowledge is seen and that it operates athigh speed corresponding to such characteristic.

Tables 1 and 2 and FIG. 9 show one example thereof. The semiconductordevice used here is what has been fabricated in the process of (H)through (L) in FIGS. 6H through 6L below which continue from the processshown in FIGS. 5A through 5G described above. It is noted that Step G inFIG. 6G corresponds to Step G in the process shown in FIG. 5G.

-   (H) The crystal silicon film formed in the process from (A)    through (F) is patterned to form an active layer of a thin film    transistor.-   (I) Next, a silicon oxide film is formed as a GI film (gate    insulating film) by using mixed gas of SiH₄+N₂O as film forming gas    by means of plasma CVD.-   (J) A heat treatment is implemented in an oxygen atmosphere    (atmospheric pressure) containing 3 volume % of HCl at 950° C. for    28 minutes. As a result, a thermal oxide film of 300 angstrom in    thickness is formed and the thickness of the crystal silicon film is    reduced to 250 angstrom. It is noted that while the reason why the    thickness of the crystal silicon film is reduced is unknown    similarly to the aforementioned case in forming the thermal oxide    film and we would have to wait for the research of the future, it is    assumed to have happened because silicon in non-crystallized state    or not crystallized completely is consumed in the formation of the    thermal oxide film. It is also noted here that the thermal oxide    film is formed on the surface of the active layer in connection with    the fact that activated oxygen molecules infiltrate into the GI    film.-   (K) An aluminum film of 4000 angstrom thickness is formed by    sputtering. It is noted that 0.18 weight % of scandium is contained    in the aluminum. Then, an anodic oxide film is formed further on the    surface of the aluminum film.-   (L) Next, a resist mask is placed and the aluminum film is patterned    to fabricate a prototype of a gate electrode.

Table 1 shows characteristics of an N-channel type TFT and Table 2 showsthat of a P-channel type TFT constructed by using the crystal siliconfilm of the present invention. In Tables 1 and 2, measurement points 1through 20 mean that they are measured by using each spot on the surfaceof one batch of the crystal silicon film fabricated as described above.As it is apparent from Table 1, when the crystal silicon film isconstructed as the N-channel type TFT, S-value is very small inparticular among the characteristics. It is around 80 mV/decade and iswithin a range of 70 to 90 mV/decade as a whole. It is so small as 72.53mV/decade especially at the measurement point 13.

The S-value (sub-threshold coefficient) is defined as an inverse numberof a maximum inclination at the build-up portion of the curve of ID-VGas shown in FIG. 8. In other words, it is understood as an increment ofgate voltage necessary for increasing drain current by one digit. Thatis, the smaller the S-value, the sharper the inclination at the build-upportion. Then, it excels in responsibility as a switching element andcan be operated at high speed.

An ideal value derived from a theoretical formula is 60 mV/decade.Although a value close to that is obtained in a transistor using amono-crystal wafer, a conventional TFT using low temperaturepoly-silicon is limited to 300 to 500 mV/decade. In view of this fact,the S-value of around 80 mV/decade of the TFT using the crystal siliconfilm of the present invention can be said as an astonishing value.

In Table 2 showing the P-channel type TFT constructed by using thecrystal silicon film of the present invention, the S-value is also verysmall in this case similarly to the case of the N-channel type TFT. Itis around 80 mV/decade and is within a range of 70 to 100 mV/decade as awhole. It is so small as 72.41 mV/decade especially at the measurementpoint 4. These values mean that it is the same with the case of theN-channel type TFT, except only of that plus (+) and minus (−) areopposite.

Beside the above, each of the characteristics (codes or signs) means asfollows. As it is apparent from Tables 1 and 2, any of thesecharacteristics shows values which can fully sustain in practical use.I_(on) is drain current which flows when the TFT is ON and it is set asI_(on)−1 when VD=1 V (1 volt) and I_(on)−2 when VD=5V. A TFT having alarger Ion value can flow more current in a short time.

I_(off) is drain current which flows when the TFT is OFF and it is setas I_(off)−1 when VD=1 V (1 volt) and I_(off)−2 when VD=5V. If currentflows when the TFT is OFF, electric power is consumed that much, so thatit is very important to minimize I_(off). If I_(off) is large, therearises a problem that charge held in a liquid crystal flows out byI_(off). I_(on)/I_(off)−1 (or I_(on)/I_(off)−2) is a ratio betweenI_(on)−1 and I_(off)−1 and represents how many digits the ON currentdiffers from the OFF current. The greater the I_(on)/I_(off), the betterthe switching characteristic is. It is important also in increasingcontrast on a display panel.

Vth is a parameter generally called as a threshold voltage and isdefined as voltage when a TFT is switched to ON for example. Valueswithin the table are those obtained by means of root ID extrapolation bysetting as the objects of evaluation when VD=5. If Vth is large, voltageapplied to a gate electrode has to be set high, so that driving voltageas well as power consumption increase. μFE represents a mobility offield effect. It is a parameter indicating the mobility of carriers. ATFT having a large μFE can be said to be suitable for high-speedoperation. As it is apparent from Tables 1 and 2, any of thesecharacteristics show values which can fully sustain in practical use.

FIGS. 9 a and 9 b are graphs of the I_(D)−V_(G) characteristic drawn byselecting typical values from the above-mentioned actually measureddata. FIG. 9 a shows the case of the N-channel type TFT and FIG. 9 bshows the case of the P-channel type TFT. In the both cases, VD=1V. Thehorizontal axis within FIGS. 9 a and 9 b represents gate voltage (V) andthe vertical axis represents drain current (A). The scale unit of thevertical axis is “1E−13” through “1E−01”, i.e. within a range of 1×10⁻¹³through 1×10⁻¹ A (Ampere).

Seeing the case of the N-channel type TFT in FIG. 9 a at first, it canbe seen that the inclination of the build-up portion of the ID-VG curve,i.e. the curve at the linear region, is very sharp. It shows thecharacteristic corresponding to that the S-value described above issmall as it is and indicates that the TFT has an excellentresponsibility as a switching element and can be operated at high speed.Further, while the range between −6 V to −0.5 V of gate voltage in FIG.9 a corresponds to I_(off) within Table 1 described above, it can beseen that the drain current flowing when the TFT is OFF is very smalland that the TFT has another excellent quality also in this aspect.

Next, seeing FIG. 9 b, the curve at the linear region is very sharp andthe drain current flowing when the TFT is OFF is very small also in thecase of the P-channel type TFT. It also has an excellent characteristicssimilarly to the N-channel type TFT described above. It is noted that asfor such technological significance, only the codes (signs9 of plus (+)and minus (−) are different as compared to the case of the N-channeltype TFT.

FIGS. 11 a and 11 b show oscillograms obtained by constructing a ringoscillator by building a circuit in which the N-channel type TFT and theP-channel type TFT described above are combined and by operating it.This circuit works such that the N-channel type TFT and the P-channeltype TFT compensate their operation each other in the same time, i.e.such that when one TFT discharges electric charge, the other TFT sucksthe electric charge.

FIG. 10 is a diagrammatic view for explaining FIGS. 11 a and 11 b. WhenFIG. 10 is seen as a whole, the waveform on the + side of theoscillating waveform is related mainly with the operation of theN-channel type TFT and the waveform on the − side is related mainly withthe operation of the P-channel type TFT. Accordingly, when it isoscillated in 152.0 MHz, 252.9 MHz or the like and if the oscillatingwaveform keeps symmetry on the + and − sides, it means that theN-channel type TFT and the P-channel type TFT operate symmetrically withrespect to the frequency and operate normally with the similarcharacteristics.

Then, seeing the oscillograms in FIGS. 11 a and 11 b, the oscillatingwaveform is a sine wave, no distortion is seen in the linearity and theyare symmetrical both vertically and horizontally. Thus, it can be seenthat the inventive crystal silicon film shows the excellentcharacteristics even when it is applied as the N-channel type orP-channel type and that substantially there is no difference in thecharacteristics between the both.

A model which can explain the above-mentioned phenomena andcharacteristics may be considered as follows. At first, as seen in thephotographs of the electron microscope shown in FIGS. 3 and 4, thesilicon semiconductor thin film composing the TFT composed of thecrystal silicon film obtained by the present invention has a structurein which the crystal continues in the specific direction. According todetailed observation by means of the electron microscope, it has beenconfirmed that the lattice structures are continuing in the specificdirection.

From the above-mentioned observation result, this state is construed asa state in which mono-crystals are continuing in the specific directionleaving a predetermined space therebetween. It is then understood,naturally, that carries can readily move in the direction to which thelattice structures continue. That is, it is assumed that the channelregion is composed of countless thin and long channels in the TFT usingthe crystal silicon film obtained by the present invention.

Here, while linear grain boundaries observed in the photographs in FIGS.3 and 4 partition very small channels, no state in which impuritiessegregate specifically at the grain boundary is seen.

Such grain boundary contains mismatch and distortion of crystalstructures and an energy level there is considered to be higher than theother regions. Accordingly, it is assumed to have a function ofrestricting the move of the carriers in the direction to which thecrystal structures are continuing. If such narrow and small channels areformed, a range of osmosis of influence from the source and drainregions exerted to the inside of the small channels are considered tobecome small corresponding to the narrowness.

As it is analogized how electromagnetic wave expands in a space wherethere is no obstacle for example, the electrical influence is assumed toexpand, ideally, isotropically In two- or three-dimension. In view ofsuch fact, because the countless small and narrow channels are formed inthe TFT using the crystal silicon film obtained by the presentinvention, it can be understood that the influence from the source anddrain regions to the channel is suppressed in each individual smallchannel and that it suppresses the short-channel effect as a whole.

FIGS. 12 and 13 are graphs of measured values of gate current of aplanar type thin film transistor which the inventors et. al. have madein trial by using the crystal silicon film in which the concentration ofthe metal element which promotes the crystallization of silicon isreduced in the processes of numerous studies and tests conducted untilreaching to the present invention. FIG. 12 is different from FIG. 13 inthat whether thermal oxidation is used or plasma CVD is used in forminga gate insulating film.

That is, FIG. 12 shows measured values obtained when the gate insulatingfilm has been formed by the thermal oxidation and FIG. 13 shows measuredvalues obtained when the gate insulating film has been formed by theplasma CVD. In FIGS. 12 and 13, the horizontal axis represents the gatecurrent and the vertical axis represents a number of measured samples. Aquartz substrate is used as the substrate here. Further, an active layeris formed by holding nickel element in contact on the surface of theamorphous silicon film and by crystallizing by a heat treatment of fourhours at 640° C. The thermal oxide film is formed within an oxygenatmosphere at 950° C.

It can be seen from FIG. 12 that the values of gate current vary largelydepending on the samples. It shows that there is dispersion in thequality of the gate insulating film. Meanwhile, as shown in FIG. 13,there is less dispersion of the gate currents and its value is extremelysmall in the thin film transistor in which the gate insulating film hasbeen formed by the plasma CVD. The reason why the difference of themeasured values shown in FIGS. 12 and 13 appears may be explained asfollows.

That is, nickel element is sucked from the active layer to the thermaloxide film during when the thermal oxide film is formed in the samplesin which the gate insulating film is formed by the thermal oxide film.As a result, nickel element which hampers the insulation comes to existwithin the thermal oxide film. The existence of the nickel elementincreases a value of current leaked within the gate insulating film andvaries that value.

This fact is supported by SIMS (secondary ion mass spectrometry) and bymeasuring the concentration of nickel element within the gate insulatingfilm of the samples whose measured values have been obtained in FIGS. 12and 13. That is, it was confirmed that while nickel element of more thanthe level of 10¹⁷ cm⁻³ is measured within the gate insulating filmformed by the thermal oxidation, the concentration of nickel elementwithin the gate insulating film formed by the plasma CVD is less thanthe level of 10¹⁶ cm⁻³. It is noted that the concentration of impuritydescribed in the present specification is defined as the minimum valueof the measured values measured by the SIMS.

The point described above is one example of findings which the inventorset. al. have obtained in the process of numerous studies and experimentsconducted until reaching to the present invention and the presentinvention is based on such findings. That is, the thermal oxide film isformed on the surface of the crystal silicon film obtained by utilizingthe metal element which promotes the crystallization of silicon togetter the metal element within the thermal oxide film and to reduce theconcentration of the metal element or to eliminate the metal elementwithin the crystal silicon film as a result.

Regardless whether it is the amorphous silicon film or the crystalsilicon film, the metal element is normally a harmful substance within asemiconductor device composed of a thin film transistor (TFT and thelike) using the silicon film, so that the metal element needs to beeliminated from the silicon film as much as possible. According to thepresent invention, such metal element which promotes the crystallizationof silicon may be eliminated or reduced very effectively after using itin the formation of the crystal silicon film. Thereby, the presentinvention allows a semiconductor device having such excellentcharacteristics to be obtained.

The main aspects of the present invention described above in (1) through(6) are as follows.

According to one aspect of the invention described above in (1) and (2),an amorphous silicon film is formed at first. Then, the amorphoussilicon film is crystallized by the effect of the metal element typifiedby nickel which promotes the crystallization of silicon to obtain acrystal silicon film. This crystallization is carried out by heattreatment. In the state after the heat treatment, the metal elementwhich has been intentionally introduced is contained in certain highconcentration within the crystal silicon film.

Another heat treatment is implemented to the crystal silicon film in theabove-mentioned state in the oxidizing atmosphere to form a thermaloxide film on the surface of the crystal silicon film. At this time, themetal element is gettered to the thermal oxide film, thus reducing theconcentration of the metal element or eliminating the metal elementwithin the crystal silicon film. Then, the thermal oxide film which hasgettered the metal element is eliminated.

The crystal silicon film which has a high crystallinity and from whichthe metal element has been eliminated or in which the concentration ofthe metal element is low can be obtained through those steps. Thethermal oxide film described above may be eliminated by using bufferhydrofluoric acid or other hydrofluoric etchant. This process foreliminating the thermal oxide film is implemented in the same manner inthe process for eliminating the thermal oxide film in each aspect of theinvention described below.

According to one aspect of the invention described above in (3), anactive layer of a thin film transistor is formed by implementingpatterning after the above-mentioned steps. That is, this active layeris formed by the crystal silicon film from which the metal element hasbeen eliminated or in which the concentration of the metal element islow. Then, a thermal oxide film which composes at least a part of a gateinsulating film is formed on the surface of the active layer by means ofthermal oxidation to compose a semiconductor device.

According to one aspect of the invention described above in (4), anamorphous silicon film is formed at first and a metal element whichpromotes crystallization of silicon is selectively introduced to theamorphous silicon film. There is no particular limit how to selectivelyintroduce the metal element to the amorphous silicon film and variousmethods may be adopted, such as 1) introducing to one end portion of theamorphous silicon film, 2) introducing to one end portion of theamorphous silicon film leaving a space, and 3) introducing in dots onthe whole surface of the amorphous silicon film leaving spacestherebetween. Thereby, the crystal is grown by a first heat treatment ina direction parallel to the film from the region to which the metalelement has been selectively introduced.

Next, a thermal oxide film is formed on the surface of the region wherethe crystal has been grown by implementing a second heat treatmentwithin an oxidizing atmosphere. At this time, the metal element isgettered to the thermal oxide film by the action of oxygen within theoxidizing atmosphere and the concentration of the metal element withinthe crystal silicon film is reduced or the metal element is eliminatedtherefrom. Further, the thermal oxide film is eliminated and an activelayer of the semiconductor device is formed by using the region fromwhich the thermal oxide film has been eliminated. FIG. 6 is a processflowchart schematically showing the main point of the arrangementdescribed above.

In any of these aspects, the temperature of the second heat treatment ispreferable to be higher than that of the first heat treatment and it ispreferable to anneal in a plasma atmosphere containing oxygen andhydrogen after eliminating the thermal oxide film. Further, theconcentration of oxygen contained in the amorphous silicon film ispreferable to be 5×10¹⁷ cm⁻³ to 2×10¹⁹ cm⁻³.

Thus, according to one aspect of the present invention, there isprovided a semiconductor device having the crystal silicon filminterposed between first and second oxide films and containing a metalelement which promotes crystallization of silicon and which isdistributed in high concentration near the interfaces with the firstand/or second oxide film within the crystal silicon film. According toone aspect of this semiconductor device, the first oxide film is asilicon oxide film or silicon oxynitride film formed on a glasssubstrate or quartz substrate, the crystal silicon film composes anactive layer of a thin film transistor and the second oxide film may becomposed of a silicon oxide film or silicon oxynitride film which formsa gate insulating film.

Similarly to the case described above, according to one aspect of thepresent invention, there is provided a semiconductor device comprisingan underlying layer made from an oxide film; a crystal silicon filmformed on the underlying layer; and a thermal oxide film formed on thecrystal silicon film; wherein the crystal silicon film contains a metalelement which promotes crystallization of silicon; the metal elementwhich promotes the crystallization of silicon is distributed in highconcentration near the interface with the underlying layer and/orthermal oxide film; and the thermal oxide film composes at least a partof a gate insulating film of a thin film transistor.

The main aspects of the present invention described above in (7) through(12) are as follows.

According to one aspect of the invention described above in (7), anamorphous silicon film is formed at first. Then, a metal element whichpromotes crystallization of silicon is intentionally introduced to theamorphous silicon film and the amorphous silicon film is crystallized bya first heat treatment to obtain a crystal silicon film. The metalelement is contained in the crystal silicon film in the state after theheat treatment. Then, the metal element existing within the crystalsilicon film is eliminated or reduced by implementing a second heattreatment within an oxidizing atmosphere containing a halogen element.

At this time, the metal element is gettered to the thermal oxide film bythe actions of oxygen, halogen and halogen and oxygen, thus reducing theconcentration of the metal element within the crystal silicon film oreliminating the metal element. Then, after eliminating the thermal oxidefilm formed there, another thermal oxide film is formed on the surfaceof the region from which the thermal oxide film has been eliminated byimplementing another thermal oxidation.

According to one aspect of the invention described above in (8), anamorphous silicon film is formed at first. Then, a metal element whichpromotes crystallization of silicon is intentionally introduced to theamorphous silicon film and the amorphous silicon film is crystallized bya first heat treatment to obtain a crystal silicon film. Next, the metalelement existing within the crystal silicon film is eliminated orreduced by implementing a second heat treatment within an oxidizingatmosphere containing a halogen element to form a thermal oxide film onthe surface of the crystal silicon film and by causing the thermal oxidefilm to getter the metal element. Then, after eliminating the thermaloxide film formed there, another thermal oxide film is formed on thesurface of the domain from which the thermal oxide film has beeneliminated by implementing another thermal oxidation.

According to one aspect of the invention described above in (9), anamorphous silicon film is formed at first. Then, a metal element whichpromotes crystallization of silicon is intentionally introduced to theamorphous silicon film and the amorphous silicon film is crystallized bya first heat treatment to obtain a crystal silicon film. Next, the metalelement existing within the crystal silicon film is eliminated orreduced by implementing a second heat treatment within an oxidizingatmosphere containing a halogen element. After eliminating a thermaloxide film formed there, an active layer of a thin film transistor isformed by implementing patterning and another thermal oxide film whichcomposes at least a part of a gate insulating film is formed on thesurface of the active layer by means of thermal oxidation. FIG. 6 is aprocess flow chart schematically showing the main part of thearrangement described above.

According to one aspect of the invention described above in (10), anamorphous silicon film is formed at first. Then, a metal element whichpromotes crystallization of silicon is selectively introduced to theamorphous silicon film. As modes for selectively introducing the metalelement to the amorphous silicon film, various methods may be adopted,such as 1) introducing to one end portion of the amorphous silicon film,2) introducing to one end portion of the amorphous silicon film leavinga space, and 3) introducing in dots on the whole surface of theamorphous silicon film leaving spaces therebetween. Thereby, the crystalis grown by a first heat treatment in a direction parallel to the filmfrom the region to which the metal element has been selectivelyintroduced.

Next, a thermal oxide film is formed on the surface of the region wherethe crystal has been grown by implementing a second heat treatmentwithin an oxidizing atmosphere containing halogen element. The thermaloxide film is eliminated and an active layer of the semiconductor deviceis formed by using the region from which the thermal oxide film has beeneliminated. This active layer is formed of the crystal silicon film fromwhich the metal element has been eliminated or the crystal silicon filmin which the concentration of the metal element is low.

In the methods for fabricating the semiconductor device described above,an atmosphere in which one or a plurality of types of gases selectedfrom HCl, HF, HBr, Cl₂, F₂ and Br₂ is added to O₂ atmosphere may be usedas the oxidizing atmosphere containing the halogen element.

Further, the temperature of the second heat treatment is preferable tobe higher than that of the first heat treatment and it is preferable toanneal in the plasma atmosphere containing oxygen and hydrogen aftereliminating the thermal oxide film. Further, the concentration of oxygencontained within the amorphous silicon film is preferable to be 5×10¹⁷cm⁻³ to 2×10¹⁹ cm⁻³.

By the fabrication method described above, there is provided thesemiconductor device in (11), i.e. the semiconductor device having acrystal silicon film interposed between first and second oxide films,wherein the crystal silicon film contains hydrogen and a halogen elementas well as a metal element which promotes crystallization of silicon andthe metal element is distributed in high concentration near theinterfaces with the first and/or second oxide film within the crystalsilicon film.

In the semiconductor device, the halogen element is contained in highconcentration in the first oxide film and/or near the interface betweenthe first oxide film and the crystal silicon film and the halogenelement is contained in high concentration near the interface in thecrystal silicon film with the second oxide film. Further, the firstoxide film is a silicon oxide film or silicon oxynitride film formed ona glass substrate or quartz substrate, the crystal silicon film composesan active layer of a thin film transistor and the second oxide film maybe composed of a silicon oxide film or silicon oxynitride film whichforms a gate insulating film.

Similarly to the case described above, according to one aspect of thepresent invention described above in (12), there is provided asemiconductor device comprising an underlying layer made from an oxidefilm; a crystal silicon film formed on the underlying layer; and athermal oxide film formed on the crystal silicon film; wherein thecrystal silicon film contains a metal element which promotescrystallization of silicon and hydrogen and halogen element; the metalelement which promotes the crystallization of silicon is distributed inhigh concentration near the interface with the underlying layer and/orthermal oxide film; and the halogen element is distributed in highconcentration near the interface with the underlying layer and/orthermal oxide film; and the thermal oxide film composes at least a partof a gate insulating film of a thin film transistor.

The main aspects of the present invention described above in (13)through (17) are as follows.

According to one aspect of the invention described above in (13), anamorphous silicon film is formed at first. Then, a metal element whichpromotes crystallization of silicon is intentionally introduced to theamorphous silicon film and the amorphous silicon film is crystallized bya first heat treatment to obtain a crystal silicon film. After that,laser light or intense light is irradiated to the crystal silicon film.Next, a second heat treatment is implemented in an oxidizing atmospherecontaining a halogen element to eliminate or reduce the metal elementexisting within the crystal silicon film. Then, a thermal oxide filmformed there is eliminated and another thermal oxide film is formed onthe surface of the region from which the thermal oxide film has beeneliminated by implementing another thermal oxidation.

According to one aspect of the invention described above in (14), anamorphous silicon film is formed at first. Then, a metal element whichpromotes crystallization of silicon is intentionally introduced to theamorphous silicon film and the amorphous silicon film is crystallized bya first heat treatment to obtain a crystal silicon film. After that,laser light or intense light is irradiated to the crystal silicon filmto diffuse the metal element existing within the crystal silicon film.

Next, a second heat treatment is implemented in an oxidizing atmospherecontaining a halogen element to getter the metal element existing withinthe crystal silicon film to a thermal oxide film to be formed. Then, thethermal oxide film formed there is eliminated and another thermal oxidefilm is formed on the surface of the region from which the thermal oxidefilm has been eliminated by implementing another thermal oxidation.

According to one aspect of the invention described above in (15), anamorphous silicon film is formed at first. Then, a metal element whichpromotes crystallization of silicon is intentionally introduced to theamorphous silicon film. As modes for selectively introducing the metalelement to the amorphous silicon film, various methods may be adopted,such as 1) introducing to one end portion of the amorphous silicon film,2) introducing to one end portion of the amorphous silicon film leavinga space, and 3) introducing in dots on the whole surface of theamorphous silicon film leaving spaces therebetween. Thereby, the crystalis grown by a first heat treatment described below in a directionparallel to the film from the region to which the metal element has beenselectively introduced.

The first heat treatment is implemented to the amorphous silicon film togrow crystal in a direction parallel to the film from a region of theamorphous silicon film into which the metal element has beenintentionally and selectively introduced. After that, laser light orintense light is irradiated to diffuse the metal element existing withinthe region where the crystal has grown. A second heat treatment isimplemented within an oxidizing atmosphere containing a halogen elementto cause the metal element existing within the region where the crystalhas grown to be gettered to a thermal oxide film to be formed. Then, thethermal oxide film formed here is eliminated and another thermal oxidefilm is formed on the surface of the region from which the thermal oxidefilm has been eliminated by implementing another thermal oxidation.

In the aspects of the invention described above in (13) through (15), itis preferable to conduct the second heat treatment in temperature above600° C. and below 750° C. and it is preferable to form the gateinsulating film by using said another thermal oxide film. Further, inthose aspects of the invention, the atmosphere in which one or aplurality of types of gases selected from HCl, HF, HBr, Cl₂, F₂ and Br₂is added to O₂ atmosphere is used as the oxidizing atmosphere containingthe halogen element. In these inventions, the temperature of the secondheat treatment is preferable to be higher than that of the first heattreatment.

Further, it is possible to anneal in the plasma atmosphere containingoxygen and hydrogen after eliminating the thermal oxide film in theseinventions. Further, the concentration of oxygen contained within theamorphous silicon film is 5×10¹⁷ cm⁻³ to 2×10¹⁹ cm⁻³.

According to one aspect of the invention described above in (16), anamorphous silicon film is formed at first. Then, a metal element whichpromotes crystallization of silicon is intentionally introduced to theamorphous silicon film and the amorphous silicon film is crystallized bya first heat treatment to obtain a crystal silicon film. Next, an activelayer of the semiconductor device is formed by patterning the crystalsilicon film and laser light or intense light is irradiated to theactive layer. After that, a second heat treatment is implemented withinan oxidizing atmosphere containing a halogen element to eliminate orreduce the metal element existing within the active layer. Then, athermal oxide film formed here is eliminated and another thermal oxidefilm is formed on the surface of the active layer by implementinganother thermal oxidation.

According to one aspect of the invention described above in (17), anamorphous silicon film is formed at first. Then, a metal element whichpromotes crystallization of silicon is intentionally introduced to theamorphous silicon film and the amorphous silicon film is crystallized bya first heat treatment to obtain a crystal silicon film. Next, an activelayer of the semiconductor device is formed by patterning the crystalsilicon film and laser light or intense light is irradiated to theactive layer. After that, a second heat treatment is implemented withinan oxidizing atmosphere containing a halogen element to eliminate orreduce the metal element existing within the active layer. Then, athermal oxide film formed here is eliminated and another thermal oxidefilm is formed on the surface of the active layer by implementinganother thermal oxidation. At this time, the active layer is formed soas to have an inclined shape in which an angle formed between a sideface and an underlying face is 20° to 50°.

In the aspects of the invention described above in (16) and (17), thegate insulating film may be formed by utilizing said another thermaloxide film. Further, it is preferable that the temperature of the firstand second heat treatments is below 750° C. Preferably, the atmospherein which one or a plurality of types of gases selected from HCl, HF,HBr, Cl₂, F₂ and Br₂ is added to O₂ atmosphere is used as the oxidizingatmosphere containing the halogen element.

In these inventions, the temperature of the second heat treatment ispreferable to be higher than that of the first heat treatment. Further,it is possible to anneal in the plasma atmosphere containing oxygen andhydrogen after eliminating the thermal oxide film in these inventions.Still more, the concentration of oxygen contained within the amorphoussilicon film is 5×10¹⁷ cm⁻³ to 2×10¹⁹ cm⁻³.

The main aspects of the present invention described above in (18)through (22) are as follows.

According to one aspect of the invention described above in (18), anamorphous silicon film is formed at first. Then, a metal element whichpromotes crystallization of silicon is intentionally introduced to theamorphous silicon film and the amorphous silicon film is crystallized bya first heat treatment to obtain a crystal silicon film. Next, laserlight or intense light is irradiated to the crystal silicon film and asecond heat treatment is implemented within an oxidizing atmosphere toeliminate or reduce the metal element existing within the crystalsilicon film. Then, a thermal oxide film formed in that step iseliminated and another thermal oxide film is formed on the surface ofthe region from which the thermal oxide film has been eliminated byimplementing another thermal oxidation.

According to one aspect of the invention described above in (19), anamorphous silicon film is formed at first. Then, a metal element whichpromotes crystallization of silicon is intentionally introduced to theamorphous silicon film and the amorphous silicon film is crystallized bya first heat treatment to obtain a crystal silicon film. Next, laserlight or intense light is irradiated to the crystal silicon film todiffuse the metal element existing within the crystal silicon film.After that, a second heat treatment is implemented within an oxidizingatmosphere to getter the metal element existing within the crystalsilicon film to a thermal oxide film to be formed. Then, the thermaloxide film formed in that step is eliminated and another thermal oxidefilm is formed on the surface of the region from which the thermal oxidefilm has been eliminated by implementing another thermal oxidation.

According to one aspect of the invention described above in (20), anamorphous silicon film is formed at first. Then, a metal element whichpromotes crystallization of silicon is intentionally and selectivelyintroduced to the amorphous silicon film. As modes for selectivelyintroducing the metal element to the amorphous silicon film, variousmethods may be adopted, such as 1) introducing to one end portion of theamorphous silicon film, 2) introducing to one end portion of theamorphous silicon film leaving a space, and 3) introducing in dots onthe whole surface of the amorphous silicon film leaving spacesherebetween. Thereby, the crystal is grown by the following first heattreatment in a direction parallel to the film from the region to whichthe metal element has been selectively introduced.

That is, the first heat treatment is implemented to the amorphoussilicon film to grow the crystal in the direction parallel to the filmfrom the region to which the metal element has been intentionally andselectively introduced. Next, laser light or intense light is irradiatedto the crystal silicon film to diffuse the metal element existing withinthe crystal silicon film. After that, a second heat treatment isimplemented within an oxidizing atmosphere to getter the metal elementexisting within the crystal silicon film to a thermal oxide film to beformed. Then, the thermal oxide film formed in that step is eliminatedand another thermal oxide film is formed on the surface of the regionfrom which the thermal oxide film has been eliminated by implementinganother thermal oxidation.

In the aspects of the invention described above in (19) and (20), thesecond heat treatment is preferably conducted in temperature above 600°C. and below 750° C. The gate insulating film may be formed by utilizingsaid another thermal oxide film. Further, it is preferable that thetemperature of the second heat treatment is higher than that of thefirst heat treatment. Still more, in these inventions, it is possible toanneal in the plasma atmosphere containing oxygen and hydrogen aftereliminating the thermal oxide film. Further, preferably, theconcentration of oxygen contained within the amorphous silicon film is5×10¹⁷ cm⁻³ to 2×10¹⁹ cm⁻³.

According to one aspect of the invention described above in (21), anamorphous silicon film is formed at first. Then, a metal element whichpromotes crystallization of silicon is introduced intentionally to theamorphous silicon film and the amorphous silicon film is crystallized bya first heat treatment to obtain a crystal silicon film. Next, thecrystal silicon film is patterned to form an active layer of thesemiconductor device and laser light or intense light is irradiated tothe active layer. After that, a second heat treatment is implementedwithin an oxidizing atmosphere to eliminate or reduce the metal elementexisting within the active layer. A thermal oxide film formed in thatstep is eliminated and another thermal oxide film is formed on thesurface of the active layer by implementing another thermal oxidation.

According to one aspect of the invention described above in (22), anamorphous silicon film is formed at first. Then, a metal element whichpromotes crystallization of silicon is introduced intentionally to theamorphous silicon film and the amorphous silicon film is crystallized bya first heat treatment to obtain a crystal silicon film. Next, thecrystal silicon film is patterned to form an active layer of thesemiconductor device and laser light or intense light is irradiated tothe active layer. After that, a second heat treatment is implementedwithin an oxidizing atmosphere to eliminate or reduce the metal elementexisting within the active layer. A thermal oxide film formed in thatstep is eliminated and another thermal oxide film is formed on thesurface of the active layer by implementing another thermal oxidation.At this time, the active layer is formed so as to have an inclined shapein which an angle formed between a side face and an underlying facethereof is 20° to 50°.

In the aspects of the invention described above in (21) and (22), thegate insulating film may be formed by utilizing said another thermaloxide film. The second heat treatment is preferably conducted intemperature above 600° C. and below 750° C. Further, it is preferablethat the temperature of the second heat treatment is higher than that ofthe first heat treatment. Still more, in these inventions, it ispreferable to anneal in the plasma atmosphere containing oxygen andhydrogen after eliminating the thermal oxide film. Preferably, theconcentration of oxygen contained within the amorphous silicon film is5×10¹⁷ cm⁻³ to 2×10¹⁹ cm⁻³.

The main aspects of the inventions described above in (23) through (25)are as follows.

According to one aspect of the invention described above in (23), anamorphous silicon film is formed on a substrate having an insulatingsurface and a metal element which promotes crystallization of silicon isintentionally introduced to the amorphous silicon film. Then, theamorphous silicon film is crystallized by a first heat treatment in thetemperature range of 750° C. to 1100° C. to obtain a crystal siliconfilm. The crystal silicon film is patterned to form an active layer ofthe semiconductor device.

After that, a second heat treatment is implemented within an oxidizingatmosphere containing a halogen element to eliminate or reduce the metalelement existing within the crystal silicon film. A thermal oxide filmformed in the previous step is eliminated and another thermal oxide filmis formed after eliminating the thermal oxide film by implementinganother thermal oxidation. At this time, the heat treatments areimplemented such that a temperature of the second heat treatment ishigher than that of the first heat treatment.

According to one aspect of the invention described above in (24), anamorphous silicon film is formed on a substrate having an insulatingsurface and a metal element which promotes crystallization of silicon isintentionally introduced to the amorphous silicon film. Then, theamorphous silicon film is crystallized by a first heat treatment in thetemperature range of 750° C. to 1100° C. to obtain a crystal siliconfilm. The crystal silicon film is patterned to form an active layer ofthe semiconductor device. After that, a second heat treatment isimplemented within an oxidizing atmosphere containing a halogen elementto getter the metal element existing within the active layer to athermal oxide film to be formed. The thermal oxide film formed in theprevious step is then eliminated and another thermal oxide film isformed after eliminating the thermal oxide film by implementing anotherthermal oxidation. At this time, the heat treatments are implementedunder the condition that a temperature of the second heat treatment ishigher than that of the first heat treatment.

According to one aspect of the invention described above in (25), anamorphous silicon film is formed on a substrate having an insulatingsurface and a metal element which promotes crystallization of silicon isintentionally introduced to the amorphous silicon film. As modes forselectively introducing the metal element to the amorphous silicon film,various methods may be adopted, such as 1) introducing to one endportion of the amorphous silicon film, 2) introducing to one end portionof the amorphous silicon film leaving a space, and 3) introducing indots on the whole surface of the amorphous silicon film leaving spacestherebetween. Thereby, the crystal is grown by the following first heattreatment in a direction parallel to the film from the region to whichthe metal element has been selectively introduced.

That is, the crystal is grown by the first heat treatment in thetemperature range of 750° C. to 1100° C. in a direction parallel to thefilm from the region to which the metal element has been intentionallyand selectively introduced. Then, the crystal silicon film is patternedto form an active layer of the semiconductor device by using the regionin which the crystal has grown in the direction parallel to the film.After that, a second heat treatment is implemented within an oxidizingatmosphere containing a halogen element to getter the metal elementexisting within the active layer to a thermal oxide film to be formed.The thermal oxide film formed in the previous step is then eliminatedand another thermal oxide film is formed after eliminating the thermaloxide film by implementing another thermal oxidation. At this time, theheat treatments are implemented under the condition that a temperatureof the second heat treatment is higher than that of the first heattreatment.

In the aspects of the invention described above in (23) through (25),preferably, a quartz substrate is used as the substrate for forming theamorphous silicon film and the gate insulating film is formed byutilizing said another thermal oxide film. Further, in these inventions,it is possible to anneal in the plasma atmosphere containing oxygen andhydrogen after eliminating the thermal oxide film. Still more,preferably, the concentration of oxygen contained within the amorphoussilicon film is 5×10¹⁷ cm⁻³ to 2×10¹⁹ cm⁻³.

The main aspects of the present invention described above in (26)through (29) are as follows.

According to one aspect of the invention described above in (26), anamorphous silicon film is formed at first. Then, a metal element whichpromotes crystallization of silicon is held in contact on the surface ofthe amorphous silicon film and the amorphous silicon film iscrystallized by a first heat treatment to obtain a crystal silicon film.Next, a second heat treatment is implemented in the temperature range of500° C. to 700° C. within an atmosphere containing oxygen, hydrogen andfluorine to form a thermal oxide film on the surface of the crystalsilicon film. Then, the thermal oxide film is eliminated.

According to one aspect of the invention described above in (27), anamorphous silicon film is formed at first. Then, a metal element whichpromotes crystallization of silicon is held in contact on the surface ofthe amorphous silicon film and the amorphous silicon film iscrystallized by a first heat treatment to obtain a crystal silicon film.Next, a second heat treatment is implemented in the temperature range of500° C. to 700° C. within an atmosphere containing oxygen, hydrogen,fluorine and chlorine to form a thermal oxide film on the surface of thecrystal silicon film. Then, the thermal oxide film is eliminated.

According to one aspect of the invention described above in (28), anamorphous silicon film is formed at first. Then, a metal element whichpromotes crystallization of silicon is held in contact on the surface ofthe amorphous silicon film and the amorphous silicon film iscrystallized by a first heat treatment to obtain a crystal silicon film.Next, a second heat treatment is implemented within an atmospherecontaining fluorine and/or chlorine to form a wet oxide film on thesurface of the crystal silicon film. Then, the oxide film is eliminated.

In the aspects of the invention described above in (26) through (28)above, preferably the concentration of the metal element within theoxide film is higher than that of the metal element within the crystalsilicon film. Further, it is preferable to contain more than 1% andbelow an explosion limit of hydrogen in the atmosphere in which thesecond heat treatment is implemented. Further, it is preferable toimplement the first heat treatment in a reducing atmosphere. Laser lightmay be irradiated to the crystal silicon film after the first heattreatment.

According to one aspect of the invention described above in (29), thereis provided a semiconductor device having a silicon film having acrystallinity, characterized in that the silicon film contains a metalelement which promotes crystallization of silicon in concentration of1×10¹⁶ cm⁻³ to 5×10¹⁸ cm⁻³, fluorine atoms in concentration of 1×10¹⁵cm⁻³ to 1×10²⁰ cm⁻³, and hydrogen atoms in concentration of 1×10¹⁷ cm⁻³to 1×10²¹ cm⁻³. In this invention, preferably, the silicon film isformed on the insulating film and fluorine atoms exist in highconcentration near the interface between the insulating film and thesilicon film.

The main aspects of the invention described above in (30) through (33)are as follows.

According to one aspect of the invention described above in (30), anamorphous silicon film is formed at first and the amorphous silicon filmis crystallized to form a crystal silicon film. Next, this crystalsilicon film is heated within an oxidizing atmosphere to which fluorinecompound gas is added to grow a thermal oxide film on the surface of thecrystal silicon film. Then, the thermal oxide film on the surface of thecrystal silicon film is eliminated. After that, an insulating film isdeposited on the surface of the crystal silicon film to complete thefabrication of the semiconductor device.

According to one aspect of the invention described above in (31), thereis provided a method for fabricating a thin film transistor on asubstrate having an insulating surface. An amorphous silicon film isformed at first and the amorphous silicon film is crystallized to form acrystal silicon film. Next, this crystal silicon film is heated withinan oxidizing atmosphere to which fluorine compound gas is added to growa thermal oxide film on the surface of the crystal silicon film. Then,the thermal oxide film on the surface of the crystal silicon film iseliminated. After that, the crystal silicon film is shaped to form anactive layer of the thin film transistor and an insulating film isdeposited on the surface of the active layer to form a gate insulatingfilm at least on the surface of a channel region. Further, a gateelectrode is formed on the surface of the gate insulating film andimpurity ions which give a conductive type are injected into the activelayer by using the gate electrode as a mask to form a source and a drainin a manner of self-alignment.

According to one aspect of the invention described above in (32), anamorphous silicon film is formed and the amorphous silicon film iscrystallized by irradiating laser light to form a crystal silicon film.Next, this crystal silicon film is heated within an oxidizing atmosphereto which fluorine compound gas is added to grow a thermal oxide film onthe surface of the crystal silicon film. Then, the thermal oxide film onthe surface of the crystal silicon film is eliminated. After that, aninsulating film is deposited on the surface of the crystal silicon filmto complete the fabrication of the semiconductor device.

According to one aspect of the invention described above in (33), thereis provided a method for fabricating a thin film transistor on asubstrate having an insulating surface. An amorphous silicon film isformed at first and the amorphous silicon film is crystallized to form acrystal silicon film. Next, laser light is irradiated to the crystalsilicon film and the is heated within an oxidizing atmosphere to whichfluorine compound gas is added to grow a thermal oxide film on thesurface of the crystal silicon film. Then, the thermal oxide film on thesurface of the crystal silicon film is eliminated.

Next, the crystal silicon film is shaped to form an active layer of thethin film transistor, an insulating film is deposited on the surface ofthe active layer to form a gate insulating film at least on the surfaceof a channel region and a gate electrode is formed on the surface of thegate insulating film. Further, a source and a drain are formed in amanner of self-alignment by injecting impurity ions which give aconductive type to the active layer by using the gate electrode as amask. Thus, the semiconductor device is fabricated.

In the aspects of the invention described above in (30) through (33),preferably, the thickness of the thermal oxide film is 200 to 500angstrom and the metal element is doped to the amorphous silicon filmafter forming the amorphous silicon film in concentration of 1×10¹⁶ to5×10¹⁹ atoms/cm³. Further, while a metal element is preferable to use informing the crystal silicon film, at least one or more types of elementsselected from Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Cu and Au may be used asthe metal element similarly to the aspects of the invention describedabove.

The specific nature of the invention, as well as other objects, uses andadvantages thereof, will clearly appear from the following descriptionof the preferred embodiments and the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a picture (optical microphotograph: 450 times) showing amicro-structure of a crystal silicon film obtained by the presentinvention;

FIG. 2 is a picture (optical microphotograph: 450 times) showing amicrostructure of the crystal silicon film obtained by the presentinvention;

FIG. 3 is a picture (TEM: 50,000 times) showing the micro-structure ofthe crystal silicon film obtained by the present invention;

FIG. 4 is a picture (TEM: 250,000 times) showing the micro-structure ofthe crystal silicon film obtained by the present invention;

FIGS. 5A though 5G are schematic diagrams showing one example of typicalmodes of manufacturing steps of the crystal silicon film of the presentinvention;

FIGS. 6A through 6F are schematic diagrams showing one example oftypical modes of manufacturing steps of a semiconductor device using thecrystal silicon film of the present invention;

FIGS. 7 a and 7 b are diagrammatic views showing the form of crystalgrowth supposed from the result observed from a number ofmicrophotographs of the crystal silicon film of the present invention;

FIG. 8 is a schematic graph for explaining a sub-threshold value(S-value) and others of the semiconductor device;

FIGS. 9 a and 9 b are graphs showing various characteristics such as thesub-threshold characteristic (S value) of the semiconductor device usingthe crystal silicon film of the present invention;

FIG. 10 is a schematic chart for explaining a characteristic of a ringoscillator into which a circuit in which an N-channel type TFT and aP-channel type TFT are combined is built;

FIGS. 11 a and 11 b are charts showing oscilloscopes (oscillatingwaveforms) by the ring oscillator into which the circuit in which theN-channel type TFT and the P-channel type TFT are combined by using thecrystal silicon film of the present invention is built;

FIG. 12 is a graph showing measured values of gate current of a planartype thin film transistor using the crystal silicon film of the presentinvention;

FIG. 13 is a graph showing measured values of gate current of the planartype thin film transistor using the crystal silicon film of the presentinvention;

FIG. 14 is a graph showing a measured result of concentrationdistribution of Ni element in a direction of section of the film at themoment when a thermal oxide film is formed after crystallizing anamorphous silicon film by using Ni;

FIG. 15 is a graph showing a measured result of concentrationdistribution of Ni element in the direction of section of the film atthe moment when the thermal oxide film is formed after crystallizing theamorphous silicon film by using Ni;

FIG. 16 is a graph showing a measured result of concentrationdistribution of Cl in the direction of section of the film at the momentwhen the thermal oxide film is formed after crystallizing the amorphoussilicon film by using Ni;

FIG. 17 is a graph showing a measured result of concentrationdistribution of Ni element in the direction of section of the film atthe moment when the thermal oxide film is formed after crystallizing theamorphous silicon film by using Ni;

FIG. 18 is a graph showing a measured result of concentrationdistribution of Ni element in the direction of section of the film atthe moment when the thermal oxide film is formed after crystallizing theamorphous silicon film by using Ni;

FIG. 19 is a graph showing a measured result of concentrationdistribution of Cl in the direction of section of the film at the momentwhen the thermal oxide film is formed after crystallizing the amorphoussilicon film by using Ni;

FIGS. 20A through 20E are diagrams showing fabrication steps accordingto a fourth embodiment;

FIGS. 21A through 21E are diagrams showing fabrication steps accordingto a ninth embodiment;

FIGS. 22A through 22E are diagrams showing fabrication steps accordingto a tenth embodiment;

FIGS. 23A through 23E are diagrams showing fabrication steps accordingto a twelfth embodiment;

FIGS. 24A through 24F are diagrams showing fabrication steps accordingto a thirteenth embodiment;

FIGS. 25A through 25E are diagrams showing fabrication steps accordingto a sixteenth embodiment;

FIGS. 26A through 26E are diagrams showing fabrication steps accordingto a 21-st embodiment;

FIGS. 27A through 27E are diagrams showing fabrication steps accordingto a 22-nd embodiment;

FIGS. 28A through 28E are diagrams showing fabrication steps accordingto a 24-th embodiment;

FIGS. 29A through 29F are diagrams showing fabrication steps accordingto a 25-th embodiment;

FIGS. 30A through 30E are diagrams showing fabrication steps accordingto a 28-th embodiment;

FIGS. 31A through 31E are diagrams showing fabrication steps accordingto a 30-th embodiment;

FIGS. 32A through 32E are diagrams showing fabrication steps accordingto a 31-st embodiment;

FIGS. 33A through 33E are diagrams showing fabrication steps accordingto a 33-rd embodiment;

FIGS. 34A through 34F are diagrams showing fabrication steps accordingto a 34-th embodiment;

FIGS. 35A through 35F are diagrams showing fabrication steps accordingto a 37-th embodiment;

FIGS. 36A and 36B are diagrammatic view for explaining a phenomenon whenlaser light is irradiated to the surface of the crystal silicon film;

FIGS. 37A through 37E are diagrams showing fabrication steps accordingto a 39-th embodiment;

FIGS. 38A through 38E are diagrams showing fabrication steps accordingto a 41-st embodiment;

FIGS. 39A through 39E are diagrams showing fabrication steps accordingto a 42-nd embodiment;

FIGS. 40A through 40E are diagrams showing fabrication steps accordingto a 44-th embodiment;

FIGS. 41A through 41F are diagrams showing fabrication steps accordingto a 45-th embodiment;

FIGS. 42A through 42F are diagrams showing fabrication steps accordingto a 48-th embodiment;

FIGS. 43A through 43E are diagrams showing fabrication steps accordingto a 50-th embodiment;

FIGS. 44A through 44E are diagrams showing fabrication steps accordingto a 52-nd embodiment;

FIGS. 45A through 45E are diagrams showing fabrication steps accordingto a 53-rd embodiment;

FIGS. 46A through 46E are diagrams showing fabrication steps accordingto a 54-th embodiment;

FIGS. 47A through 47F are diagrams showing fabrication steps accordingto a 55-th embodiment;

FIGS. 48A through 48E are diagrams showing fabrication steps accordingto a 58-th embodiment;

FIGS. 49A through 49E are diagrams showing fabrication steps accordingto a 60-th embodiment;

FIGS. 50A through 50E are diagrams showing fabrication steps accordingto a 62-nd embodiment;

FIGS. 51A through 51E are diagrams showing fabrication steps accordingto a 63-rd embodiment;

FIGS. 52A through 52F are diagrams showing fabrication steps accordingto a 66-th embodiment;

FIGS. 53A through 53F are diagrams showing fabrication steps accordingto a 67-th embodiment;

FIGS. 54A through 54E are diagrams showing fabrication steps accordingto the 67-th embodiment;

FIGS. 55A and 55B are diagrams showing fabrication steps according to a68-th embodiment;

FIGS. 56A through 56E are diagrams showing fabrication steps accordingto the 68-th embodiment;

FIGS. 57A through 57E are diagrams showing fabrication steps accordingto a 69-th embodiment;

FIGS. 58A through 58D are diagrams showing fabrication steps accordingto the 69-th embodiment;

FIGS. 59A through 59D are diagrams showing fabrication steps accordingto the 60-th embodiment;

FIGS. 60A through 60C are drawings showing several examples amongvarious applied examples of the semiconductor device of the presentinvention; and

FIGS. 60D through 60F are drawings showing several examples amongvarious applied examples of the semiconductor device of the presentinvention.

DESCRIPTION OF PREFERRED EMBODIMENTS

While the present invention will be explained below in detail based onthe preferred embodiments thereof, it is to be understood that thepresent invention will not be confined to those embodiments. At first,embodiments of effects of eliminating or reducing a metal element withina crystal silicon film which has been crystallized by an action of themetal element will be described. After that, embodiments whichcorrespond to the aspects of the invention described above in (1)through (33) will be described.

[First Embodiment]

FIG. 14 shows a measured result of the distribution of concentration ofnickel element in a direction of section of a crystal silicon filmobtained by utilizing nickel as the metal element. This measurement wascarried out by SIMS (secondary ion mass spectrometry). Steps forfabricating the sample from which these measured values have beenobtained will be explained below briefly.

A silicon oxide film is formed as an underlying film in a thickness of4000 angstrom on a quartz substrate and then an amorphous silicon filmis formed in a thickness of 500 angstrom by low pressure thermal CVD.Next, nickel element is introduced to the amorphous silicon film byusing nickel acetate aqueous solution. Further, the amorphous siliconfilm is crystallized by a heat treatment at 650° C. for four hours toobtain a crystal silicon film. After that, another heat treatment isimplemented within an oxidizing atmosphere at 950° C. to form a thermaloxide film 500 angstrom thick.

As it is apparent from FIG. 14, the nickel element has moved from thecrystal silicon film (poly-Si film) to the thermal oxide film and iscontained in the thermal oxide film. Yet, it exists within the crystalsilicon film (poly-Si film) relatively in high concentration. It isnoted that the reason why the concentration of nickel element is high onthe surface of the thermal oxide film is considered to be measurementerrors caused by the state of the surface such as irregularities andadsorbed substances on the surface, so that it is not significant. Datanear the interface also contains errors more or less by the same reason.

[Second Embodiment]

Next, as another method for forming the thermal oxide film, a heattreatment is implemented within an oxidizing (e.g. oxygen) atmospherecontaining 3 volume % of HCl to form the thermal oxide film 500 angstromthick. FIG. 15 shows measured data of this sample. As it is apparentfrom FIG. 15, the concentration of nickel within the crystal siliconfilm (poly-Si) is reduced further and instead of that, the concentrationof nickel within the thermal oxide film is increased. This means thatthe nickel element has been sucked out (i.e. gettered) to the thermaloxide film.

The difference between FIG. 14 and FIG. 15 is only whether HCl has beencontained within the atmosphere in forming the thermal oxide film.Accordingly, it can be concluded that HCl is related significantly withthe above-mentioned effect of gettering, beside oxygen. Further, becauseno gettering effect is confirmed by H (hydrogen) which is a component ofHCl, it can be seen that, more accurately, the gettering effect asindicated by the difference between FIG. 14 and FIG. 15 can be obtainedby action of Cl (chlorine).

The crystal silicon film in which the nickel concentration has beenreduced can be obtained by eliminating the thermal oxide film which hasgettered the nickel element. FIG. 16 is a graph showing the distributionof concentration of Cl element in a sample fabricated under the samecondition with that of the sample from which the data in FIG. 15 hasbeen obtained. As it is apparent from FIG. 16, the Cl element isconcentrated near the interface between the crystal silicon film and thethermal oxide film.

[Third Embodiment]

A third embodiment relates to a case when an amorphous silicon filmformed by plasma CVD is utilized instead of the amorphous silicon filmwhich is the starting film of the crystal silicon film from which thedata described in the first and second embodiments have been obtained.The other fabrication conditions are the same with those in the firstembodiment. Because the quality of the amorphous silicon film formed bythe plasma CVD is different from that of the amorphous silicon filmformed by the low pressure thermal CVD, the action of gettering exertedafter forming the crystal silicon film is also different.

FIG. 17 shows measured data of a sample in which the thermal oxide filmhas been formed in an oxygen atmosphere at 950° C. As it is apparentfrom FIG. 17, although the nickel element has moved to the thermal oxidefilm, the nickel element exists relatively in high concentration withinthe crystal silicon film (poly-Si). It is noted that although thecondition for introducing nickel is the same, the nickel concentrationis high within the crystal silicon film (poly-Si) as compared to that inFIG. 14. It is assumed to have happened because the nickel element canreadily diffuse within the film because the quality of the amorphoussilicon film formed by the plasma CVD is not dense and has many defects.

The above-mentioned fact may be seen from another point of view asfollows. That is, while a very thin oxide film has been formed on thesurface of the amorphous silicon film by means of UV oxidation beforeapplying the nickel acetate aqueous solution, there is a possibilitythat the thickness of the oxide film have become different by receivingan influence of difference of the quality of the underlying amorphoussilicon film. It can be understood that because the amount of nickelelement which diffuses within the silicon film is different depending onthe difference of the thickness in this case, its effect has appeared asthe difference between FIG. 14 and FIG. 17.

FIG. 18 shows data when 1 volume % of HCl is contained in oxygen as theatmosphere in which the thermal oxide film is formed. As it, is apparentfrom FIG. 18, the nickel concentration within the crystal silicon filmdrops further as compared to the data shown in FIG. 17. In contrary tothat, the nickel concentration within the thermal oxide film hasincreased.

This fact means that the nickel element has been gettered to the thermaloxide film by the action of chlorine. Thus, the nickel element existingwithin the silicon film may be gettered effectively to the thermal oxidefilm by forming the thermal oxide film within the oxidizing atmospherecontaining chlorine. Accordingly, the crystal silicon film in which thenickel concentration is reduced may be obtained by eliminating thethermal oxide film which has gettered the nickel element.

A graph shown in FIG. 19 show a result obtained by studying theconcentration of chlorine within a sample obtained under the samefabrication condition with that of the sample from which the data shownin FIG. 18 has been obtained. As it is apparent from FIG. 19, chlorineexists in high concentration near the interfaces between the underlyingfilm and the crystal silicon film and between the crystal silicon filmand the thermal oxide film. While FIG. 19 corresponds to FIG. 16, thereason why the chlorine concentration is distributed as shown in FIG. 19is considered to happen because the amorphous silicon film which is thestarting film has been formed by the plasma CVD and the quality thereofis not dense.

As it is also apparent from FIG. 19, the nickel concentration is apt tobe high near the interface between the underlying film and the crystalsilicon film. It is understood to happen as a result that nickel hasbeen gettered to the underlying film by the action of chlorine existingnear the interface with the underlying film (or within the underlyingfilm). Such phenomenon is considered to be obtained also when a halogenelement is added to the underlying film.

[Fourth Embodiment]

A fourth embodiment relates to a case when a crystal silicon film isobtained on a glass substrate by utilizing nickel element. At first, thecrystal silicon film having a high crystallinity is obtained by theaction of nickel element. Next, a thermal oxide film is formed on thecrystal silicon film by thermal oxidation. At this time, the nickelelement remaining in the crystal silicon film is gettered to the thermaloxide film. Then, the thermal oxide film containing the nickel elementin high concentration as a result of the gettering is eliminated.Thereby, the crystal silicon film which has the high crystallinity andin which the concentration of nickel element is low is obtained on theglass substrate.

FIGS. 20A through 20E are diagrams showing the fabrication process inthe fourth embodiment. At first, a silicon oxynitride film 2 is formedas an underlying film in a thickness of 3000 angstrom on the glasssubstrate of Corning 1737 (distortion point: 667° C.). While the siliconoxynitride film is formed by using plasma CVD using silane, N₂O gas andoxygen as original gases or plasma CVD using TEOS gas and N₂O gas, theformer is used in the present embodiment.

The silicon oxynitride film has a function of suppressing the diffusionof impurities from the glass substrate in the later steps (seeing fromthe level of fabrication of a semiconductor, a glass substrate containsa large amount of impurities). A silicon oxide film may be used as theunderlying film instead of the silicon oxynitride film. It is noted thatalthough the silicon nitride film is the most suitable to obtain thefunction for suppressing the diffusion of the impurities in maximum, thesilicon nitride film is not practical because it is peeled off from theglass substrate due to the influence of stress.

It is also important to increase the hardness of the underlying film 2as much as possible. It is concluded from the fact that the harderthe-hardness of the underlying film (i.e. the smaller the etching ratethereof), the higher the reliability is in an endurance test of the thinfilm transistor obtained in the end. Although the reason thereof isunknown in detail, it is assumed to be caused by the effect of blockingthe impurities from the glass substrate in the fabrication process ofthe thin film transistor.

It is also effective to contain a small amount of halogen elementtypified by chlorine in the underlying film 2. Thereby, the metalelement which promotes crystallization of silicon which exists withinthe semiconductor layer may be gettered by the halogen element in thelater step. It is also effective to add a hydrogen plasma treatmentafter forming the underlying film and it is effective to implement aplasma treatment in an atmosphere in which oxygen and hydrogen aremixed. These treatments are effective in eliminating carbon componentwhich is adsorbed on the surface of the underlying film and in enhancingthe characteristic of interface with a semiconductor film formed later.

Next, an amorphous silicon film 3, which turns out to be a crystalsilicon film later, is formed in a thickness of 500 angstrom by the lowpressure thermal CVD. The reason why the low pressure thermal CVD isused is because thereby, the quality of the crystal silicon filmobtained later is better, i.e. the film quality is denser in concrete.Beside the low pressure thermal CVD, the plasma CVD or the like may beused. The amorphous silicon film fabricated here is desirable to have5×10¹⁷ cm⁻³ to 2×10¹⁹ cm⁻³ of concentration of oxygen within the film.It is because oxygen plays an important role in the later step ofgettering the metal element which promotes crystallization of silicon.

However, it must be careful here because the crystallization of theamorphous silicon film is hampered if the oxygen concentration is higherthan the above-mentioned range of concentration. The concentration ofother impurities such as those of nitrogen and carbon is preferred to below to the utmost. In concrete, the concentration must be below 2×10¹⁹cm⁻³. The thickness of the amorphous silicon film 3 is 1600 angstrom.The thickness of the amorphous silicon film must be thicker than athickness which is required in the end as described later.

When the amorphous silicon film 3 is crystallized by means of onlyheating, the thickness of the starting film (amorphous silicon film) 3is set at 800 angstrom to 5000 μm or preferably, 1500 to 3000 angstrom.It is uneconomical from the aspect of production cost if the thicknessis thicker than the above-mentioned thickness range because it takesmore time in forming the film. When the thickness is thinner than theabove-mentioned thickness range on the other hand, the crystal may begrown non-uniformly or the reproducibility of the process may becomebad. Thus, the state shown in FIG. 20A is obtained. Next, nickel elementis introduced to the amorphous silicon film 3 to crystallize it. Here,the nickel element is introduced by applying nickel acetate solutioncontaining 10 ppm (weight conversion) of nickel on the surface of theamorphous silicon film 3.

Beside the method of using the nickel salt solution as described above,sputtering, CVD, plasma treatment or adsorption may be used as themethod for introducing the nickel element. Among them, the method ofusing the solution is useful in that it is simple and that theconcentration of the metal element may be readily adjusted. Variousnickel salts may be used as the nickel salt and organic solvents such asalcoholic solvents and others or mixed solvent of organic water andorganic solvent may be used beside water as the solvent.

In the present embodiment, the nickel acetate solution is applied asdescribed above to form a water film 4 as shown in FIG. 20B. In thisstate, extra solution is blown out by using a spin coater not shown.Thus, the nickel element is held in contact on the surface of theamorphous silicon film 3. It is noted that it is preferable to usesolution containing no carbon and containing nickel salt, e.g. nickelsulfate solution, instead of using the nickel acetate solution, if theremained impurities in the later heating process is taken intoconsideration. It is because the nickel acetate solution contains carbonand it might be carbonized in the later heating process, thus remainingwithin the film. An amount of the nickel element to be introduced may becontrolled by adjusting the concentration of nickel salt within thesolution.

Next, a heat treatment is implemented in the temperature range from 450°C. to 650° C. in the state shown in FIG. 20C to crystallize theamorphous silicon film 3 and to obtain a crystal silicon film 5. Thisheat treatment is implemented in a reducing atmosphere. Here, the heattreatment is implemented for four hours at 620° C. within a nitrogenatmosphere containing 3 volume % of hydrogen. The reason why thereducing atmosphere is adopted in the crystallization step in a way ofthe heat treatment is to prevent oxides from being created in the stepof the heat treatment and more concretely, to suppress nickel fromreacting with oxygen and NiOx from being created on the surface of thefilm or within the film.

By the way, oxygen couples with nickel and contributes a lot ingettering nickel in the later gettering step. However, it has been foundthat if oxygen couples with nickel in the above-mentioned stage of thecrystallization, it hampers the crystallization. Accordingly, it isimportant to suppress the oxides from being created to the utmost in thecrystallization step in a way of heating. The concentration of oxygenwithin the atmosphere for implementing the heat treatment for thecrystallization has to be in an order of ppm, or preferably, less than 1ppm.

While inert gases such as nitrogen and argon or their mixed gas may beused as the gas which occupies the most of the atmosphere forimplementing the heat treatment for the crystallization, nitrogen isused in the present embodiment. The lower limit of the heatingtemperature for the crystallization is preferred to be more than 450° C.from the aspects of its effectiveness and reproducibility. On the otherhand, the upper limit thereof is preferred to be less than thedistortion point of the used glass substrate. Because the Corning 1737glass substrate whose distortion point is 667° C. is used in the presentembodiment, its upper limit is set at 650° C., leaving some margin.

In this regard, the heating temperature may be increased up to about900° C. or more if a quartz substrate is used as the substrate. In thiscase, a crystal silicon film having a higher crystallinity may beobtained in a shorter time. Thus, the crystal silicon film 5 is obtainedas shown in FIG. 20C. Another heat treatment is implemented afterobtaining the crystal silicon film 5 to form a thermal oxide filmcontaining nickel element. This heat treatment is implemented within anatmosphere of 100% of oxygen.

FIG. 20D is a diagram for explaining this heat treatment step. This stepis carried out to eliminate the nickel element (or another metal elementwhich promotes the crystallization of silicon) which has been introducedintentionally for the crystallization in the initial stage from thecrystal silicon film 5. This heat treatment is implemented at atemperature higher than that of the heat treatment implemented for thecrystallization described above. It is an important condition foreffectively implementing the gettering of nickel element.

This heat treatment is implemented in the temperature range from 550° C.to 1050° C. or preferably from 600° C. to 980° C. upon meeting theabove-mentioned condition. It is because no effect is obtained if thetemperature is below 600° C. and a fixture formed by quartz distorts ora burden is placed upon equipments if it exceeds 1050° C. (in thissense, it is preferable to be less than 980° C.). Further, the upperlimit of the heating temperature is limited by the distortion point ofthe glass substrate to be used. It must be careful not to implement theheat treatment in a temperature above the distortion point of the glasssubstrate to be used because, otherwise, it is deformed. Also, the lowerlimit depends upon a pressure at which the annealing is performed and avapor pressure of the halogen compound of the material to be gettered.That is, when the vapor pressure of the halogen compound is smaller thanthe pressure of the annealing atmosphere, the gettering efficiency isnot so high. For example, the vapor pressure of nickel chloride is 38.9mmHg at 541° C. and 820.6 mmHg at 994° C. Accordingly, when theannealing is performed at the atmospheric pressure (760 mmHg), theeffect of the gettering is significantly increased when the temperatureis 994° C.

Because the Corning 1737 glass substrate whose distortion point is 667°C. is used in the present embodiment, the heating temperature is set at640° C. Then, a thermal oxide film 6 as shown in FIG. 20D is formed byimplementing the heat treatment under such condition. Here, the thermaloxide film 6 is formed in a thickness of 200 angstrom by implementingthe heat treatment for 12 hours. Because the thermal oxide film 6 isformed, the thickness of the crystal silicon film 3 reaches to about1500 angstrom. When the heating temperature is 600° C. to 750° C. in theheat treatment, the treatment time (heating time) is set at 10 hours to48 hours or typically at 24 hours. It is noted that if the heatingtemperature is 750° C. to 900° C., the treatment time is set at 5 hoursto 24 hours or typically at 12 hours.

Further, when the heating temperature is 900° C. to 1050° C., thetreatment time is set at 1 hour to 12 hours or typically at 6 hours.These treatment times are set adequately depending on the thickness ofthe oxide film to be obtained as a matter of course. In this step,nickel element is gettered to the thermal oxide film 6 to be formed. Inthe gettering, oxygen existing within the crystal silicon film, besidethat existing within the oxygen atmosphere, plays an important role.That is, oxygen couples with nickel, thus producing nickel oxide, andthe nickel element is gettered to the thermal oxide film 6 in this form.

If the concentration of oxygen is too much, it becomes the factor ofhampering the crystallization of the amorphous silicon film 3 in thecrystallization step shown in FIG. 20C as described above. However, theexistence thereof plays an important role in the process of getteringnickel as described above. Accordingly, it is important to control theconcentration of oxygen existing within the amorphous silicon film, thestarting film. Through this step, the nickel element within the crystalsilicon film 5 may be eliminated or its concentration may be reduced ascompared to its initial concentration.

Because the nickel element is gettered to the oxide film to be formed inthe above-mentioned step, naturally the nickel concentration within theoxide film becomes high as compared to other domains. Further, it hasbeen observed that the concentration of nickel element is apt to be highnear the interface of the crystal silicon film 5 with the oxide film 6.It is considered to happen because the domain where the gettering mainlytakes place is on the side of the oxide film near the interface betweenthe silicon film and the oxide film. The gettering proceeding near theinterface is considered to be caused by the existence of stress anddefects or organic substances near the interface.

After forming the thermal oxide film 6 containing nickel in highconcentration, it is eliminated. While the thermal oxide film 6 may beeliminated by means of wet etching or dry etching using bufferhydrofluoric acid (or other hydrofluorite (hydrofluoric) etchant), theformer is used in the present embodiment. Thus, a crystal silicon film 7in which the concentration of nickel has been reduced or from whichnickel has been eliminated is obtained as shown in FIG. 20E. Becausenickel element is contained near the surface of the obtained crystalsilicon film 7 relatively in high concentration, it is effective toadvance the above-mentioned etching of the thermal oxide film 6 toover-etch, more or less, the surface of the crystal silicon film 7.

[Fifth Embodiment]

A fifth embodiment relates to a case when laser light is irradiatedfurther after obtaining the crystal silicon film by the heat treatmentshown in FIG. 20C in the arrangement shown in the fourth embodiment topromote the crystallization thereof. When the temperature of the heattreatment shown in FIG. 20C is low or when the treatment time is short,i.e. when the heating temperature or the heating time is restricted bythe reason of the fabrication process, there is a possibility that therequired crystallinity cannot not be obtained. In such a case, therequired high crystallinity may be obtained by implementing annealing byirradiating laser light. The irradiation of laser light allows the widthof permissible laser irradiation condition to be widened and thereproducibility thereof to be increased as compared to the case ofcrystallizing the amorphous silicon film directly.

The irradiation of laser light may be implemented after the step shownin FIG. 20C. It is also important to form the amorphous silicon film 3which is formed in FIG. 20A as the starting film in a thickness of 200to 2000 angstrom. It is because the annealing effect exerted by theirradiation of laser light becomes high when the thickness of theamorphous silicon film is thin. Although there is no specific limit onthe laser light to be used, preferably laser light in the ultravioletregion, e.g. excimer laser in the ultraviolet region is used. Inconcrete, while KrF excimer laser (wavelength: 248 nm), XeCl excimerlaser (wavelength: 308 nm) and the like may be used, the KrF excimerlaser (wavelength: 248 nm) is used in the present embodiment. Besidelaser light, the annealing may be implemented by irradiating intenselight by using an ultraviolet lamp or an infrared lamp.

[Sixth Embodiment]

A sixth embodiment relates to a case when the infrared lamp is usedinstead of the laser light in the fifth embodiment. The use of infraredray allows the silicon film to be heated selectively without heating theglass substrate so much. Accordingly, an effective heat treatment may beimplemented without giving thermal damage to the glass substrate.

[Seventh Embodiment]

A seventh embodiment relates to a case when Cu is used as the metalelement which promotes crystallization of silicon in the arrangementshown in the fourth embodiment. In case of copper element, whilesolutions such as cupric acetate [Cu(CH₃COO)₂] and cupric chloride(CuCl₂2H₂O) may be used as the solution for introducing Cu, an aqueoussolution of the cupric acetate [Cu(CH₃COO)₂] is used in the presentembodiment.

[Eighth Embodiment]

An eighth embodiment relates to a case when a quartz substrate is usedas the substrate 1 in the arrangement shown in the fourth embodiment. Inthe present embodiment, the amorphous silicon film 3, i.e. the startingfilm, is formed in a thickness of 2000 angstrom. Further, the heatingtemperature in forming the thermal oxide film in the heat treatmentshown in FIG. 20C is set at 950° C. In this case, the oxide film isformed quickly and the gettering effect cannot be fully obtained, sothat the concentration of oxygen within the atmosphere is lowered. Inconcrete, the oxygen concentration within the nitrogen atmosphere is setat 10 volume %. In this case, the treatment time is set at 300 minutes.Under such conditions, the thermal oxide film having about 500 angstromof thickness may be obtained. In the same time, the time necessary forgettering may be earned.

It is noted that if the heat treatment is implemented at 950° C. withinthe atmosphere of 100% oxygen, the thermal oxide film having a thicknessof more than 500 angstrom is obtained in about 30 minutes. In this case,nickel cannot be fully gettered, so that nickel element remainsrelatively in high concentration within the crystal silicon film 7.Accordingly, it is preferable to form the thermal oxide film byadjusting the oxygen concentration as described in the presentembodiment to earn an enough time for the gettering effect. Theutilization of this method allows the time necessary for the getteringto be set by adjusting the oxygen concentration in the atmosphere whenthe thickness and forming temperature of the thermal oxide film arechanged.

[Ninth Embodiment]

A ninth embodiment relates to a case of growing crystal in the formdifferent from that in the fourth embodiment. That is, the presentembodiment relates to a method of growing the crystal in a directionparallel to the substrate, i.e. a method called lateral growth, byutilizing the metal element which promotes crystallization of silicon.FIGS. 21A through 21E show the fabrication process according to theninth embodiment. At first, a silicon oxynitride film is formed as anunderlying film 9 in a thickness of 3000 angstrom on the Corning 1737glass substrate 8. A quartz substrate may be used of course instead ofthe glass substrate.

Next, an amorphous silicon film 10 which is the starting film of acrystal silicon film is formed in a thickness of 2000 angstrom by lowpressure thermal CVD. The thickness of the amorphous silicon film ispreferable to be less than 2000 angstrom as described before. It isnoted that plasma CVD may be used instead of the low pressure thermalCVD. Next, a silicon oxide film not shown is formed in a thickness of1500 angstrom and is patterned to form a mask 11. An opening is createdon the mask in a domain 12. The amorphous silicon film 10 is exposed atthe domain where the opening 12 is created.

The opening 12 has a thin and long rectangular shape in the longitudinaldirection from the depth to the front side of the figure. Preferably,the width of the opening 12 is 20 μm or more. The length thereof in thelongitudinal direction may be determined arbitrarily. Then, the nickelacetate aqueous solution containing 10 ppm of nickel element in terms ofweight is applied as described in the fourth embodiment and the extrasolution is removed by implementing spin drying by using a spinner notshown. Thus, the solution is held in contact on the exposed surface ofthe amorphous silicon film 10 as indicated by a dot line 13 in FIG. 21A.

Next, a heat treatment is implemented at 640° C. for four hours in anitrogen atmosphere containing 3 volume % of hydrogen and in whichoxygen is minimized. Then, crystal grows in the direction parallel tothe substrate as indicated by the reference numeral 14 in FIG. 21B. Thiscrystal growth advances from the domain of the opening 12 to whichnickel element has been introduced to the surrounding part. This crystalgrowth in the direction parallel to the substrate will be referred to aslateral growth throughout the present specification.

It is possible to advance this lateral growth across more than 100 μmunder the conditions of the ninth embodiment. Then, a crystal siliconfilm 15 having the domain in which the crystal has thus grown laterallyis obtained. It is noted that crystal growth in the vertical directioncalled vertical growth advances from the surface of the silicon film tothe underlying interface in the domain where the opening 12 is formed.Then, the mask 11 made from the silicon oxide film for selectivelyintroducing nickel element is eliminated. Thus, the state shown in FIG.21C is obtained. In this state, the vertically grown domain, thelaterally grown domain and a domain in which no crystal has grown(having amorphous state) exist within the silicon film 15.

In this state, a heat treatment is implemented at 640° C. for four hourswithin an oxygen atmosphere. In this step, an oxide film 16 containingnickel element in high concentration is formed. In the same time, theconcentration of nickel element within the silicon film 15 may bereduced relatively. Here, the thermal oxide film 16 is formed in athickness of 200 angstrom. The thermal oxide film contains the getterednickel element in high concentration. Further, because the thermal oxidefilm 16 is formed, the thickness of the crystal silicon film 15 isreduced to about 1900 angstrom.

Next, the thermal oxide film 16 containing nickel element in highconcentration is eliminated in the same manner with the fourthembodiment. In the crystal silicon film of this state, the nickelelement has a distribution of concentration such that it exists in highconcentration toward the surface of the crystal silicon film.Accordingly, it is useful to etch the surface of the crystal siliconfilm to eliminate the domain in which the nickel element exists in highconcentration after eliminating the thermal oxide film 16. That is, thecrystal silicon film in which the nickel element concentration isreduced further may be obtained by etching the surface of the crystalsilicon film in which the nickel element exists in high concentration.

Next, patterning is implemented to form a pattern 17 formed of thelaterally grown domain. Here, it is important to form the pattern 17such that no vertically grown domain, amorphous domain nor an edgeportion of the laterally grown domain is included there, because theconcentration of nickel element is relatively high in the verticallygrown domain and the edge portion of the laterally grown domain and theamorphous domain in which no crystal has grown has inferior electricalcharacteristics. The concentration of nickel element which remainswithin the pattern 17 made from the laterally grown domain thus obtainedmay be reduced further as compared to the case shown in the fourthembodiment.

This is caused by the fact that the concentration of the metal elementcontained within the laterally grown domain is-low originally. Inconcrete, the concentration of nickel element within the pattern 17 madefrom the laterally grown domain may be readily reduced to the order of10¹⁷ cm⁻³ or less. When a thin film transistor is formed by utilizingthe laterally grown domain, a semiconductor device having a highermobility may be obtained as compared to the case when the verticallygrown domain as shown in the fourth embodiment (crystal grows verticallyon the whole surface in the case of the fourth embodiment) is utilized.

It is noted that it is useful to implement the etching process furtherafter forming the pattern shown in FIG. 21E to eliminate the nickelelement existing on the surface of the pattern. It is not effective toform a thermal oxide film for gettering after forming the pattern 17because in this arrangement, the etching advances so as to scoop outeven the under side of the crystal silicon film formed in an islandshape because the etching of the underlying film advances when thethermal oxide film is eliminated, though the gettering effect iscertainly obtained by the thermal oxide film.

Such a condition may cause breaking of wires and defective operation ofelements later. According to the present embodiment, a thermal oxidefilm 18 is formed after forming the pattern 17. This thermal oxide film18 is what becomes a part of a gate insulating film later inconstructing a thin film transistor and is not eliminated, though it hasthe gettering effect.

[Tenth Embodiment]

A tenth embodiment relates to a case of fabricating a thin filmtransistor disposed in a pixel region of an active matrix type liquidcrystal display or an active matrix type EL display by utilizing theinventive crystal silicon film.

FIGS. 22A through 22E show the fabrication process according to thetenth embodiment. At first, while the crystal silicon film may be formedon the glass substrate through the process shown in the fourth or ninthembodiment, the method of the fourth embodiment is used in the presentembodiment. When the crystal silicon film has been obtained in thearrangement shown in the fourth embodiment, the state shown in FIG. 22Ais obtained by patterning the crystal silicon film. In FIG. 22A, thereference numeral (20) denotes a glass substrate, (21) an underlyingfilm, and (22) an active layer formed of the crystal silicon film. Afterobtaining the state shown in FIG. 22A, a plasma treatment is implementedwithin a reduced pressure atmosphere in which oxygen and hydrogen aremixed. The plasma is formed by high-frequency discharge.

Organic substances existing on the surface of the exposed active layer22 is removed by the above-mentioned plasma treatment. Specifically, theorganic substances adsorbing on the surface of the active layer areoxidized by oxygen plasma and the oxidized organic substances arereduced and vaporized by hydrogen plasma. Thus the organic substancesexisting on the surface of the exposed active layer 22 are removed. Theremoval of the organic substances is very effective in suppressing fixedcharge from existing on the surface of the active layer 22. That is,because the fixed charge caused by the existence of organic substanceshampers the operation of the device and renders the characteristicsthereof instable, it is very useful to remove it.

After removing the organic substances, thermal oxidation is implementedwithin an oxygen atmosphere at 640° C. to form a thermal oxide film 19of 100 angstrom thick. This thermal oxide film has a high interfacialcharacteristic with a semiconductor layer and composes a part of a gateinsulating film later. Thus, the state shown in FIG. 22A is obtained.After that, a silicon oxynitride film 23 which composes the gateinsulating film is formed in a thickness of 1000 angstrom. While thefilm may be formed by using plasma CVD using mixed gas of oxygen, silaneand N₂O or plasma CVD using mixed gas of TEOS and N₂O, the former isused in the present embodiment. The silicon oxynitride film 23 functionsas the gate insulating film together with the thermal oxide film 19.

It is also effective to contain halogen element within the siliconoxynitride film 23. That is, it is possible to prevent the function ofthe gate insulating film as an insulating film from dropping by theinfluence of the nickel element (or another metal element which promotescrystallization of silicon) existing within the active layer 22 byfixing the nickel element by the action of the halogen element. It issignificant to use the silicon oxynitride film in that nickel elementhardly infiltrates to the gate insulating film from its dense filmquality. If nickel element infiltrates into the gate insulating film,its function as an insulating film drops, thus causing instability anddispersion of characteristics of the thin film transistor. It is notedthat a silicon oxide film which is normally used may be also used forthe gate insulating film.

After forming the silicon oxynitride film 23 which functions as the gateinsulating film, an aluminum film (not shown) which functions as a gateelectrode later is formed by sputtering. 0.2 weight % of scandium isincluded within the aluminum film to suppress hillock and whisker frombeing produced in the later process. The hillock and whisker mean thatabnormal growth of aluminum occurs by heating, thus forming needle orprickle-like projections.

After forming the aluminum film, a dense anodic oxide film not shown isformed. The anodic oxide film is formed by using ethylene glycolsolution containing 3 weight % of tartaric acid as electrolyte. That is,the anodic oxide film having the dense film quality is formed on thesurface of the aluminum film by setting the aluminum film as the anodeand platinum as the cathode and by anodizing within this electrolyte.The thickness of the anodic oxide film not shown having the dense filmquality is around 100 angstrom. This anodic oxide film plays a role ofenhancing the adhesiveness with a resist mask to be formed later. It isnoted that the thickness of the anodic oxide film may be controlled byadjusting voltage applied during the anodization.

Next, the resist mask 25 is formed and the aluminum film is patterned soas to have a pattern 24. The state shown in FIG. 22B is thus obtained.Here, another anodization is implemented. In this case, 3 weight % ofoxalate aqueous solution is used as electrolyte. A porous anodic oxidefilm 27 is formed by anodizing within this electrolyte by setting thealuminum pattern 26 as the anode. In this step, the anodic oxide film 27is formed selectively on the sides of the aluminum pattern because theresist mask 25 having the high adhesiveness exists thereabove.

The anodic oxide film may be grown up to several μm thick. The thicknessis 6000 angstrom in the present embodiment. It is noted that the rangeof growth may be controlled by adjusting an anodizing time. Next, theresist mask 25 is removed. Then, a dense anodic oxide film is formedagain. That is, the anodization is implemented again by using theethylene glycol solution containing 3 weight % of tartaric acid aselectrolyte. Then, an anodic oxide film 28 having a dense film qualityis formed because the electrolyte infiltrates into the porous anodicoxide film 27.

This dense anodic oxide film 28 is 1000 angstrom thick. The thickness iscontrolled by adjusting applied voltage. Here, the exposed siliconoxynitride film 23 and the thermal oxide film 19 are etched by utilizingdry etching. Then, the porous anodic oxide film 27 is eliminated byusing mixed acid in which acetic acid, nitric acid and phosphoric acidare mixed. Thus, the state shown in FIG. 22D is obtained.

After obtaining the state shown in FIG. 22D, impurity ions are injected.Here, P (phosphorus) ions are injected by plasma doping in order tofabricate an N-channel type thin film transistor. In this step, heavilydoped regions (30 and 34) and lightly doped regions (31 and 33) areformed because part of the remaining silicon oxide film 29 functions asa semi-permeable mask, thus blocking part of the injected ions.

Next, laser light is irradiated to activate the regions into which theimpurity ions have been injected. Intense light may be irradiatedinstead of laser light. Thus, a source region 30, a channel formingregion 32, a drain region 34 and low concentration impurity regions 31and 33 are formed in a manner of self-alignment. The one designated bythe reference numeral 33 in FIG. 22D is the region called the LDD(lightly doped drain). It is noted that when the dense anodic oxide film28 is formed as thick as 2000 angstrom or more, an offset gate regionmay be formed on the outside of the channel forming region 32 by itsthickness.

Although the offset gate region is formed also in the presentembodiment, it is not shown in FIG. 22 because its size is small, itscontribution due to the existence thereof is small and the figure mightotherwise become complicated. Next, a silicon oxide film or a siliconnitride film or their laminated film is formed as an interlayerinsulating film 35. The silicon oxide film is used in the presentembodiment. It is noted that the interlayer insulating film may beconstructed by forming a layer made from a resin material on the siliconoxide film or the silicon nitride film. Next, contact holes are createdto form a source electrode 36 and a drain electrode 37. Thus, the thinfilm transistor shown in FIG. 22E is completed.

[Eleventh Embodiment]

An eleventh embodiment is related to a method for forming the gateinsulating film 23 in the arrangement shown in the tenth embodiment.Thermal oxidation may be used as a method for forming the gateinsulating film when a quartz substrate or a glass substrate having ahigh heat resistance is used as the substrate. The thermal oxidationallows the film quality to be densified and is useful in obtaining athin film transistor having stable characteristics. That is, because anoxide film formed by the thermal oxidation is dense as an insulatingfilm and movable electric charge existing therein can be reduced, it isone of the most suitable films as a gate insulating film.

In the present embodiment, a heat treatment is implemented in anoxidizing atmosphere at 950° C. in forming the thermal oxide film. Atthis time, it is effective to mix HCl or the like into the oxidizingatmosphere because, thereby, metal element existing in the active layermay be fixed in the same time with the formation of the thermal oxidefilm. It is also effective to mix N₂O gas into the oxidizing atmosphereto form a thermal oxide film containing nitrogen component. Here, it isalso possible to obtain a silicon oxynitride film by the thermaloxidation if the mixed ratio of N₂O gas is optimized. It is noted thatthe thermal oxide film 19 needs not be formed in the case like thepresent embodiment and is not formed actually in the present embodiment.

[Twelfth Embodiment]

A twelfth embodiment relates to a case of fabricating a thin filmtransistor through a process different from that shown in the tenth andeleventh embodiments (FIG. 22). FIGS. 23A through 23E show thefabrication process according to the present embodiment. At first, whilethe crystal silicon film can be formed on the glass substrate throughthe process shown in the fourth or fifth embodiment, it is formed inaccordance to the process in the fourth embodiment. It is thenpatterned, thus obtaining the state shown in FIG. 23A.

After obtaining the state shown in FIG. 23A, a plasma treatment isimplemented within a reduced pressure atmosphere in which oxygen andhydrogen are mixed. In the state shown in FIG. 23A, the referencenumeral (39) denotes a glass substrate, (40) an underlying film, (41) anactive layer formed of the crystal silicon film and (38) a thermal oxidefilm formed again after eliminating the thermal oxide film forgettering. After obtaining the state shown in FIG. 23A, a siliconoxynitride film 42 which composes a gate insulating film is formed in athickness of 1000 angstrom.

While the film may be formed by using plasma CVD using mixed gas ofoxygen, silane and N₂O or plasma CVD using mixed gas of TEOS and N₂O,the latter is used in the present embodiment. The silicon oxynitridefilm 42 composes the gate insulating film together with the thermaloxide film 38. It is noted that a silicon oxide film may be used besidethe silicon oxynitride film. After forming the silicon oxynitride film42 which functions as the gate insulating film, an aluminum film (notshown) which functions as a gate electrode later is formed bysputtering. 0.2 weight % of scandium is included within the aluminumfilm.

After forming the aluminum film, a dense anodic oxide film not shown isformed. The anodic oxide film is formed by using ethylene glycolsolution containing 3 weight % of tartaric acid as electrolyte. That is,the anodic oxide film having the dense film quality is formed on thesurface of the aluminum film by setting the aluminum film as the anodeand platinum as the cathode and by anodizing within this electrolyte.The thickness of the anodic oxide film having the dense film quality isaround 100 angstrom. This anodic oxide film plays a role of enhancingthe adhesiveness with a resist mask to be formed later. It is noted thatthe thickness of the anodic oxide film may be controlled by adjustingvoltage applied during the anodization.

Next, the resist mask 43 is formed and the aluminum film is patterned soas to have a pattern 44. Here, another anodization is implemented. Inthis case, 3 weight % of oxalate aqueous solution is used aselectrolyte. A porous anodic oxide film 45 is formed by anodizing withinthis electrolyte by setting the aluminum pattern 44 as the anode. Inthis step, the anodic oxide film 45 is formed selectively on the sidesof the aluminum pattern because the resist mask 45 having the highadhesiveness exists thereabove. The anodic oxide film may be grown up toseveral μm thick. The thickness is 6000 angstrom in the presentembodiment. It is noted that the range of growth may be controlled byadjusting an anodizing time.

Next, after removing the resist mask 43, a dense anodic oxide film isformed again. That is, the anodization is implemented again by using theethylene glycol solution containing 3 weight % of tartaric acid aselectrolyte. Then, an anodic oxide film 46 having a dense film qualityas shown in FIG. 23C is formed because the electrolyte infiltrates intothe porous anodic oxide film 45. Here, the initial injection of impurityions is implemented. It is noted that this step may be implemented afterremoving the resist mask 43. A source region 47 and a drain region 49are formed by injecting the impurity ions. No impurity ion is injectedto a region 48. Then, the porous anodic oxide film 45 is eliminated byusing mixed acid in which acetic acid, nitric acid and phosphoric acidare mixed. Thus, the state shown in FIG. 23D is obtained.

After obtaining the state shown in FIG. 23D, impurity ions are injectedagain. The impurity ions are injected under the doping condition lighterthan that of the first injection. In this step, lightly doped regions(50 and 51) are formed and a region 52 turns out to be a channel formingregion in FIG. 23D. Then, intense light is irradiated by an ultravioletlamp to activate the regions into which the impurity ions have beeninjected. Laser light may be used instead of the intense light. Thus,the source region 47, the channel forming region 52, the drain region 49and low concentration impurity regions 50 and 51 are formed in a mannerof self-alignment.

Here, the one designated by the reference numeral 51 in FIG. 23D is theregion called the LDD (lightly doped drain). Next, a silicon oxide filmor a silicon nitride film or their laminated film is formed as aninterlayer insulating film 53. The silicon nitride film is used in thepresent embodiment. It is noted that the interlayer insulating film maybe constructed by forming a layer made from a resin material on thesilicon oxide film or the silicon nitride film. After that, contactholes are created to form a source electrode 54 and a drain electrode55. Thus, the thin film transistor shown in FIG. 23E is completed.

[Thirteenth Embodiment]

A thirteenth embodiment relates to a case when an N-channel type thinfilm transistor and a P-channel type thin film transistor are formed ina complementary manner. The formation shown in the present embodimentmay be utilized for various thin film integrated circuits integrated onan insulating surface as well as for peripheral driving circuits of anactive matrix type liquid crystal display for example.

FIGS. 24A through 24F are diagrams showing a fabrication processaccording to the present embodiment. At first, a silicon oxide film or asilicon oxynitride film is formed as an underlying film 58 on a glasssubstrate 57 as shown in FIG. 24A. It is preferable to use the siliconoxynitride film and it is used in the present embodiment. Next, anamorphous silicon film not shown is formed by the plasma CVD or lowpressure thermal CVD.

Then, after transforming the amorphous silicon film into a crystalsilicon film by the same method as shown in the fourth embodiment,nickel element is gettered by forming a thermal oxide film. Next, afterimplementing a plasma treatment within an atmosphere in which oxygen andhydrogen are mixed, the obtained crystal silicon film is patterned toobtain active layers 59 and 60. Further, thermal oxide films 56 whichcomposes gate insulating films are formed.

After thus obtaining the state shown in FIG. 24A, a silicon oxynitridefilm 61 is formed. It is noted that when quartz is used as thesubstrate, it is desirable to compose the gate insulating film only bythe thermal oxide film formed by using the above-mentioned thermaloxidation. Next, an aluminum film not shown which composes a gateelectrode is formed in a thickness of 4000 angstrom. Beside the aluminumfilm, a metal which can be anodized, such as tantalum, may be used.After forming the aluminum film, a very thin and dense anodic oxide filmis formed on the surface thereof by the method described before.

Next, a resist mask not shown is placed on the aluminum film to patternthe aluminum film. Then, anodization is implemented by setting theobtained aluminum pattern as the anode to form porous anodic oxide films64 and 65. The thickness of the porous anodic oxide films is 5000angstrom. Then, another anodization is implemented under the conditionof forming dense anodic oxide films 66 and 67. The thickness of thedense anodic oxide films 66 and 67 is 800 angstrom. Thus, the stateshown in FIG. 24B is obtained.

Then, the exposed silicon oxide film 61 and the thermal oxide film 56are eliminated by dry etching, thus obtaining the state shown in FIG.24C as a result. Next, the porous anodic oxide films 64 and 65 areeliminated by using mixed acid in which acetic acid, nitric acid andphosphoric acid are mixed. Thus, the state shown in FIG. 24D isobtained. Here, resist masks are disposed alternately to inject P(phosphorus) ions to the thin film transistor on the left side and B(boron) ions to the thin film transistor on the right side.

By injecting those impurity ions, a source region 70 and a drain region73 to which P ions are doped in high concentration, thus having N-type,are formed in a manner of self-alignment. Further, a region 71 to whichP ions are doped in low concentration, thus having weak N-type, as wellas a channel forming region 72 are formed in the same time. The reasonwhy the region 71 having the weak N-type is formed is because theremaining gate insulating film 68 exists. That is, part of P ionstransmitting through the gate insulating film 68 is blocked by the gateinsulating film 68.

By the same principle, a source region 77 and a drain region 74 havingstrong P-type are formed in a manner of self-alignment and a lowconcentration impurity domain 76 is formed in the same time. Further, achannel forming region 75 is formed in the same time. It is noted thatwhen the thickness of the dense anodic oxide films 66 and 67 is as thickas 2000 angstrom, an offset gate region may be formed in contact withthe channel forming region by that thickness.

It may be ignored in the case of the present embodiment because thedense anodic oxide films 66 and 67 are so thin as less than 1000angstrom. Then, laser light or intense light is irradiated to anneal theregion into which the impurity ions have been injected. Then, a siliconnitride film 78 and a silicon oxide film 79 are formed as interlayerinsulating films as shown in FIG. 24E. Their thickness is 1000 angstrom,respectively. It is noted that the silicon oxide film 79 needs not beformed in this case.

Here, the thin film transistor is covered by the silicon nitride film.The reliability of the thin film transistor may be enhanced by arrangingas described above because the silicon nitride film is dense and has anexcellent interfacial characteristic. Further, an interlayer insulatingfilm 80 made from a resin material is formed by means of spin coating.Here, the thickness of the interlayer insulating film 80 is 1 μm. Then,contact holes are created to form a source electrode 81 and a drainelectrode 82 of the N-channel type thin film transistor on the leftside. In the same time, a source electrode 83 and the drain electrode 82of the thin film transistor on the right side are formed. Here, theelectrode 82 is disposed in common.

Thus, the thin film transistor circuit having a CMOS structureconstructed in a complementary manner may be formed. In the formationshown in the present embodiment, the thin film transistor is covered bythe nitride film as well as the resin material. This formation allows toenhance the durability of the thin film transistor, so that movable ionsnor moisture hardly infiltrate. Further, it allows to preventcapacitance from being generated between the thin film transistor and awire when a multi-layered wire is formed.

[Fourteenth Embodiment]

A fourteenth embodiment relates to a case related to the arrangement offorming a mono-crystal domain or a domain which can be substantiallyconsidered as a mono-crystal domain by irradiating laser light to thecrystal silicon film obtained in the fourth or fifth embodiment.

At first, the crystal silicon film is obtained by utilizing the actionof nickel element as shown in the fourth embodiment. Next, KrF excimerlaser is irradiated to the film to promote crystallization thereoffurther. At this time, the mono-crystal domain or the domain which canbe substantially considered as a mono-crystal domain is formed by usinga heat treatment in the temperature range of more than 450° C. and byoptimizing the condition for irradiating laser light.

The film whose crystallization has been greatly promoted by such methodhas a domain which can be considered as a mono-crystal in which anelectron spin density measured by ESR is less than 3×10¹⁷ cm⁻³ and theconcentration of nickel element as the minimum value measured by SIMS(secondary ion mass spectrometry) is less than 3×10¹⁷ cm⁻³.Substantially, no grain boundary exists in this domain and highelectrical characteristics equivalent to a mono-crystal silicon wafercan be obtained.

Further, the domain which can be considered as a mono-crystal containshydrogen by less than 5 atom % to 1×10¹⁵ cm⁻³. This value is clarifiedby the measurement carried out by the SIMS (secondary ion massspectrometry). By fabricating a thin film transistor by utilizing themono-crystal or the domain which can be substantially considered as amono-crystal, one which is equivalent to a MOS type transistorfabricated by using a mono-crystal wafer may be obtained.

[Fifteenth Embodiment]

A fifteenth embodiment relates to a case when nickel element isintroduced directly to the surface of the underlying film in the processshown in the fourth embodiment. In this case, the nickel element is heldin contact on the lower surface of the amorphous silicon film. In thepresent embodiment, the nickel element is introduced after forming theunderlying film such that the nickel element (metal element) is held incontact on the surface of the underlying film. It is noted that besidethe method of using a solution, sputtering, CVD or adsorption may beused as the method for introducing nickel element.

[Sixteenth Embodiment]

A sixteenth embodiment relates to a case when a crystal silicon film isobtained on a glass substrate by utilizing nickel element. At first, thecrystal silicon film having a high crystallinity is obtained by theaction of nickel element. Next, a thermal oxide film containing ahalogen element is formed on the crystal silicon film by thermaloxidation. At this time, the nickel element remaining in the crystalsilicon film is gettered to the thermal oxide film by the action ofoxygen and halogen element.

Then, the thermal oxide film containing the nickel element in highconcentration as a result of the gettering is eliminated. Thereby, thecrystal silicon film which has the high crystallinity and in which theconcentration of nickel element is low is obtained on the glasssubstrate. FIGS. 25A through 25E are diagrams showing the fabricationprocess according to present embodiment.

At first, a silicon oxynitride film 85 is formed as an underlying filmin a thickness of 3000 angstrom on the glass substrate 84 of Corning1737 (distortion point: 667° C.). The silicon oxynitride film is formedby using plasma CVD using silane, N₂O gas and oxygen as original gasesin the present embodiment. It is noted that this film may be formed byusing plasma CVD using TEOS gas and N₂O gas.

The silicon oxynitride film has a function of suppressing the diffusionof impurities from the glass substrate in the later steps (seeing fromthe level of fabrication of a semiconductor, a glass substrate containsa large amount of impurities). It is noted that although the siliconnitride film is the most suitable to obtain the function for suppressingthe diffusion of the impurities in maximum, the silicon nitride film isnot practical because it is peeled off from the glass substrate due tothe influence of stress. A silicon oxide film may be used as theunderlying film instead of the silicon oxynitride film.

It is also important to increase the hardness of the underlying film 85as much as possible. It is concluded from the fact that the harder thehardness of the underlying film (i.e. the smaller the etching ratethereof), the higher the reliability is in an endurance test of the thinfilm transistor obtained in the end. It is assumed to be caused by theeffect of blocking the impurities from the glass substrate in thefabrication process of the thin film transistor.

It is also effective to contain a small amount of halogen elementtypified by chlorine in the underlying film 85. Thereby, the metalelement which promotes crystallization of silicon and exists within thesemiconductor layer may be gettered by the halogen element in the laterstep. It is also effective to add a hydrogen plasma treatment afterforming the underlying film. It is also effective to implement a plasmatreatment in an atmosphere in which oxygen and hydrogen are mixed. Thesetreatments are effective in eliminating carbon component which isadsorbed on the surface of the underlying film and in enhancing thecharacteristic of interface with a semiconductor film formed later.

Next, an amorphous silicon film 86, which turns out to be a crystalsilicon film later, is formed by the low pressure thermal CVD. Thereason why the low pressure thermal CVD is used is because thereby, thequality of the crystal silicon film obtained later is better, i.e. thefilm quality is denser in concrete. Beside the low pressure thermal CVD,the plasma CVD or the like may be used.

The amorphous silicon film fabricated here is desirable to have 5×10¹⁷cm⁻³ to 2×10¹⁹ cm⁻³ of concentration of oxygen within the film. It isbecause oxygen plays an important role in the later step of getteringthe metal element which promotes crystallization of silicon. However, itmust be careful here because the crystallization of the amorphoussilicon film is hampered if the oxygen concentration is higher than theabove-mentioned range of concentration. The concentration of otherimpurities such as those of nitrogen and carbon is preferred to be lowto the utmost. In concrete, the concentration must be below 2×10¹⁹ cm⁻³.

The thickness of the amorphous silicon film 86 is 1600 angstrom. Thethickness of the amorphous silicon film must be thicker than a thicknesswhich is required in the end as described later. When the amorphoussilicon film 86 is crystallized by means of only heating, the thicknessof the starting film (amorphous silicon film) 86 is set at 800 angstromto 5000 μm or preferably, 1500 to 3000 angstrom. It is uneconomical fromthe aspect of production cost if the thickness is thicker than theabove-mentioned thickness range because it takes more time in formingthe film. When the thickness is thinner than the above-mentionedthickness range on the other hand, the crystal may be grownnon-uniformly or the reproducibility of the process may become bad.Thus, the state shown in FIG. 25A is obtained.

Next, nickel element is introduced to the amorphous-silicon film 86 tocrystallize it. Here, the nickel element is introduced by applyingnickel acetate solution containing 10 ppm (weight conversion) of nickelon the surface of the amorphous silicon film 86.

Beside the method of using the above-mentioned solution, sputtering,CVD, plasma treatment or adsorption may be used as the method forintroducing the nickel element. Among them, the method of using thesolution is useful in that it is simple and that the concentration ofthe metal element may be readily adjusted.

The nickel acetate solution is applied as described above to form awater film (liquid film) 87 as shown in FIG. 25B. In this state, extrasolution is blown out by using a spin coater not shown. Thus, the nickelelement is held in contact on the surface of the amorphous silicon film86. It is noted that it is preferable to use nickel sulfate solution forexample, instead of using the nickel acetate, if the remained impuritiesin the later heating process is taken into consideration. It is becausethe nickel acetate contains carbon and it might be carbonized in thelater heating process, thus remaining within the film. An amount of thenickel element to be introduced may be controlled by adjusting theconcentration of nickel salt within the solution.

Next, a heat treatment is implemented in the temperature range from 450°C. to 650° C. in the state shown in FIG. 25C to crystallize theamorphous silicon film 86 and to obtain a crystal silicon film 88. It isimplemented at 640° C. in the present embodiment. This heat treatment isimplemented in a reducing atmosphere. Here, the heat treatment isimplemented for four hours at 620° C. within a nitrogen atmospherecontaining 3 volume % of hydrogen.

The reason why the reducing atmosphere is adopted in the crystallizationstep in a way of the heat treatment is to prevent oxides from beingcreated in the step of the heat treatment and more concretely, tosuppress nickel from reacting with oxygen and NiOx from being created onthe surface of the film or within the film. Oxygen couples with nickeland contributes a lot in gettering nickel in the later gettering step.However, it has been found that if oxygen couples with nickel in theabove-mentioned stage of the crystallization, it hampers thecrystallization. Accordingly, it is important to suppress the oxidesfrom being created to the utmost in the crystallization step in a way ofheating.

The concentration of oxygen within the atmosphere for implementing theheat treatment for the crystallization has to be in an order of ppm, orpreferably, less than 1 ppm. Inert gases such as argon, beside nitrogen,or their mixed gas may be used as the gas which occupies the most of theatmosphere for implementing the heat treatment for the crystallization.The lower limit of the heating temperature for the crystallization ispreferred to be more than 450° C. from the aspects of its effectivenessand reproducibility. On the other hand, the upper limit thereof ispreferred to be less than the distortion point of the used glasssubstrate. When the Corning 1737 glass substrate whose distortion pointis 667° C. is used like the present embodiment, its upper limit is setat 650° C., leaving some margin.

In this regard, the heating temperature may be increased up to about1100° C. in maximum (preferably up to about 1050° C.) if a materialhaving a high heat resistance such as quartz substrate is used as thesubstrate. In this case, a crystal silicon film having a highercrystallinity may be obtained in a shorter time. Thus, the crystalsilicon film 88 is obtained as shown in FIG. 25C.

Another heat treatment is implemented after obtaining the crystalsilicon film 88 to form a thermal oxide film containing halogen element.This heat treatment is implemented within an atmosphere containinghalogen element. This step is carried out to eliminate the nickelelement which has been introduced intentionally for the crystallizationin the initial stage from the crystal silicon film 88. This heattreatment is implemented at a temperature higher than that of the heattreatment implemented for the crystallization described above. It is animportant condition for effectively implementing the gettering of nickelelement. It is noted that although this heat treatment may beimplemented in about the same temperature in the heat treatmentimplemented for the crystallization, it is more effective to be higher.

This heat treatment is implemented in the temperature range from 550° C.to 1050° C. or preferably from 600° C. to 980° C. upon meeting theabove-mentioned condition. It is because no effect is obtained if thetemperature is below 600° C. and a fixture formed by quartz distorts ora burden is placed upon equipments if it exceeds 1050° C. (in thissense, while it is preferable to be less than 980° C., it may beimplemented in about 1100° C. if a high heat resistant fixture is used).Further, the upper limit of the heating temperature is limited by thedistortion point of the glass substrate to be used. It must be carefulnot to implement the heat treatment in a temperature above thedistortion point of the glass substrate to be used because, otherwise,it is deformed.

Because the Corning 1737 glass substrate whose distortion point is 667°C. is used in the present embodiment, the heating temperature is set at650° C. The second heat treatment is implemented in an atmosphere inwhich 5 volume % of HCl is mixed into oxygen. It is preferable to mixHCl with a ratio of 0.5% to 10% (volume %) to oxygen. It must be carefulnot to mix above this concentration because, otherwise, the surface ofthe film becomes rough with the same or more degree of irregularity withthe thickness of the film.

A thermal oxide film 89 containing chlorine as shown in FIG. 25D isformed by implementing he heat treatment under such condition. Here, thethermal oxide film 89 is formed in a thickness of 200 angstrom byimplementing the heat treatment for 12 hours. Because the thermal oxidefilm 89 is formed, the thickness of the crystal silicon film 88 reachesto about 1500 angstrom.

When the heating temperature is 600° C. to 750° C. in the heattreatment, the treatment time (heating time) is set at 10 hours to 48hours or typically at 24 hours. It is-noted that if the heatingtemperature is 750° C. to 900° C., the treatment time is set at 5 hoursto 24 hours or typically at 12 hours. Further, when the heatingtemperature is 900° C. to 1050° C., the treatment time is set at 1 hourto 12 hours or typically at 6 hours. These treatment times are setadequately depending on the thickness of the oxide film to be obtainedas a matter of course.

In this step, nickel element is gettered out of the silicon film by theaction of oxygen and the halogen element. Here, the nickel element isgettered to the thermal oxide film 89 to be formed by the action ofchlorine. In the gettering, oxygen existing within the crystal siliconfilm plays an important role. That is, the gettering of nickel elementproceeds effectively because the gettering effect caused by chlorineacts on nickel oxide formed when oxygen couples with nickel.

If the concentration of oxygen is too much, it becomes the factor ofhampering the crystallization of the amorphous silicon film 86 in thecrystallization step shown in FIG. 25C as described above. However, theexistence thereof plays an important role in the process of getteringnickel as described above. Accordingly, it is important to control theconcentration of oxygen existing within the amorphous silicon film, thestarting film. Here, Cl has been selected as the halogen element and thecase of using HCl has been shown as a method for introducing it. BesideHCl, one type or a plurality of types of mixed gas selected from HF,HBr, Cl₂, F₂, Br₂ may be used. Beside them, halogen hydride may be usedin general.

It is preferable to set the content (volume content) of those gaseswithin the atmosphere to 0.25 to 5% if it is HF, 1 to 15% if it is HBr,0.25 to 5% if it is Cl₂, 0.125 to 2.5% if it is F₂ and 0.5 to 10% if itis Br₂. If the concentration is below the above-mentioned range, nosignificant effect is obtained. Further, if the concentration exceedsthe upper limit of the above-mentioned range, the surface of the crystalsilicon film is roughened.

Through this step, the concentration of nickel element may be reduced to1/10 from the initial stage. It means that the nickel element may bereduced to 1/10 by the halogen element as compared to the case when nogettering is conducted by the halogen element. This effect may beobtained in the same manner even when another metal element whichpromotes crystallization of silicon is used. Because the nickel elementis gettered to the oxide film to be formed in the above-mentioned step,naturally the nickel concentration within the oxide film becomes high ascompared to other regions.

Further, it has been observed that the concentration of nickel elementis apt to be high near the interface between the crystal silicon film 88and the oxide film 89. It is considered to happen because the regionwhere the gettering mainly takes place is on the side of the oxide filmnear the interface between the silicon film and the oxide film. Thegettering proceeding near the interface is considered to be caused bythe existence of stress and defects near the interface.

Next, the thermal oxide film 89 containing nickel in high concentrationis eliminated. While the thermal oxide film 89 may be eliminated bymeans of dry etching or wet etching using buffer hydrofluoric acid orother hydrofluorite (hydrofluoric) etchant, the buffer hydrofluoric acidis used in the present embodiment.

Thus, a crystal silicon film 90 in which the concentration of nickel hasbeen reduced is obtained as shown in FIG. 25E. Because nickel element iscontained near the surface of the obtained crystal silicon film 90relatively in high concentration, it is effective to advance theabove-mentioned etching to over-etch, more or less, the surface of thecrystal silicon film 90.

[Seventeenth Embodiment]

A seventeenth embodiment relates to a case when laser light isirradiated by KrF excimer laser (wavelength: 248 nm) further afterobtaining the crystal silicon film by the heat treatment shown in FIG.25C in the arrangement shown in the sixteenth embodiment to promote thecrystallization thereof. That is, in the present embodiment, the crystalsilicon film 90 in which the nickel concentration has been reduced isobtained as shown in FIG. 25E by irradiating laser light after the heattreatment in FIG. 25C to implement annealing and implementing othersteps in the same manner as shown in the sixteenth embodiment.

When the temperature of the heat treatment shown in FIGS. 25B and 25C islow or when the treatment time is short, i.e. when the heatingtemperature or the heating time is restricted by the reason of thefabrication process, there is a possibility that the requiredcrystallinity cannot be obtained. In such a case, the required highcrystallinity may be obtained by implementing annealing by irradiatinglaser light. The irradiation of laser light in this case allows thewidth of permissible laser irradiation condition to be widened and thereproducibility thereof to be increased as compared to the case ofcrystallizing the amorphous silicon film directly by a laserirradiation.

The irradiation of laser light is implemented after the step shown inFIG. 25C. It is also important to form the amorphous silicon film 86which is formed in FIG. 25A as the starting film in a thickness of 200to 2000 angstrom. It is because the annealing effect exerted by theirradiation of laser light becomes high when the thickness of theamorphous silicon film is thin. It is preferable to use excimer laser inthe ultraviolet region as the laser light to be used. In concrete, KrFexcimer laser (wavelength: 248 nm), XeCl excimer laser (wavelength: 308nm) and the like may be used. Beside laser light, the annealing may beimplemented by irradiating intense light by using an ultraviolet lampfor example.

[Eighteenth Embodiment]

An eighteenth embodiment relates to a case when an infrared lamp is usedinstead of the laser light in the seventeenth embodiment. That is, inthe present embodiment, the crystal silicon film 90 in which the nickelconcentration has been reduced is obtained as shown in FIG. 25E byirradiating infrared ray after the heat treatment in FIG. 25C toimplement annealing and by implementing other steps in the same manneras shown in the sixteenth embodiment. The use of infrared ray allows thesilicon film to be heated selectively without heating the glasssubstrate so much. Accordingly, an effective heat treatment may beimplemented without giving thermal damage to the glass substrate.

[Nineteenth Embodiment]

A nineteenth embodiment relates to a case when Cu is used as the metalelement which promotes crystallization of silicon in the arrangementshown in the sixteenth embodiment. In this case, while solutions such ascupric acetate [Cu(CH₃COO)₂] and cupric chloride (CuCl₂2H₂O) may be usedas the solution for introducing Cu, the former is used in the presentembodiment. The crystal silicon film 90 in which the nickelconcentration has been reduced is obtained as shown in FIG. 25E byimplementing other steps in the same manner as shown in the sixteenthembodiment.

[20-th Embodiment]

A 20-th embodiment relates to a case when a quartz substrate is used asthe substrate 84 in the arrangement shown in the sixteenth embodiment.In the present embodiment, the amorphous silicon film 86, i.e. thestarting film, is formed in a thickness of 2000 angstrom. Further, theheating temperature in forming the thermal oxide film in the heattreatment shown in FIG. 25C is set at 950° C. In this case, the oxidefilm is formed quickly and the gettering effect cannot be fullyobtained, so that the concentration of oxygen within the atmosphere islowered. In concrete, the oxygen concentration within the nitrogenatmosphere is set at 10 volume %. Further, the heat treatment isimplemented in the atmosphere in which the concentration of HCl (volumeconcentration) to oxygen is set at 3%.

In this case, the treatment time is set at 300 minutes. Under suchconditions, the thermal oxide film having about 500 angstrom ofthickness may be obtained. In the same time, the time necessary forgettering may be earned. It is noted that if the heat treatment isimplemented at 950° C. within the atmosphere of 97% of oxygen and 3volume % of HCl, the thermal oxide film having a thickness of 500angstrom is obtained in about 30 minutes. In this case, nickel cannot befully gettered, so that nickel element remains relatively in highconcentration within the crystal silicon film 90. Accordingly, it ispreferable to form the thermal oxide film by adjusting the oxygenconcentration as described in the present embodiment to earn an enoughtime for the gettering effect. The utilization of this method allows thetime necessary for the gettering to be set by adjusting the oxygenconcentration in the atmosphere when the thickness and formingtemperature of the thermal oxide film are changed.

[21-st Embodiment]

A 21-st embodiment relates to a case of growing crystal in the formdifferent from that in the sixteenth embodiment. That is, the presentembodiment relates to a method of growing the crystal in a directionparallel to the substrate, i.e. a method called lateral growth, byutilizing the metal element which promotes crystallization of silicon.FIGS. 26A through 26E show the fabrication process according to the21-st embodiment.

At first, a silicon oxynitride film is formed as an underlying film 92in a thickness of 3000 angstrom on the Corning 1737 glass substrate 91.Next, an amorphous silicon film 93 which is the starting film of acrystal silicon film is formed in a thickness of 2000 angstrom by lowpressure thermal CVD. The thickness of the amorphous silicon film ispreferable to be less than 2000 angstrom as described before. It isnoted that plasma CVD may be used instead of the low pressure thermalCVD.

Next, a silicon oxide film not shown is formed in a thickness of 1500angstrom and is patterned to form a mask 94. An opening is created onthe mask in a region 95. The amorphous silicon film 93 is exposed at theregion where the opening 95 is created. The opening 95 has a thin andlong rectangular shape in the longitudinal direction from the depth tothe front side of the figure. Preferably, the width of the opening 95 is20 μm or more. The length thereof in the longitudinal direction may bedetermined arbitrarily.

Then, the nickel acetate aqueous solution containing 10 ppm of nickelelement in terms of weight is applied in the same manner with thesixteenth embodiment and the extra solution is removed by implementingspin drying by using a spinner not shown. Thus, the solution is held incontact on the exposed surface of the amorphous silicon film 93 asindicated by a dot line 96 in FIG. 26A.

Next, a heat treatment is implemented at 640° C. for four hours in anitrogen atmosphere containing 3 volume % of hydrogen and in whichoxygen is minimized. Then, crystal grows in the direction parallel tothe substrate as indicated by the reference numeral 97 in FIG. 26B. Thiscrystal growth advances from the region of the opening 95 to whichnickel element has been introduced to the surrounding part. This crystalgrowth in the direction parallel to the substrate will be referred to aslateral growth throughout the present specification.

It is possible to advance this lateral growth across more than 100 μmunder the conditions shown in the present embodiment. Then, a crystalsilicon film 98 having the region in which the crystal has thus grownlaterally is obtained. It is noted that crystal growth in the verticaldirection called vertical growth advances from the surface of thesilicon film to the underlying interface in the region where the opening95 is formed. Then, the mask 94 made from the silicon oxide film forselectively introducing nickel element is eliminated. Thus, the stateshown in FIG. 26C is obtained. In this state, the vertically grownregion, the laterally grown region and a region in which no crystal hasgrown (having amorphous state) exist within the silicon film 98.

In this state, a heat treatment is implemented at 650° C. for four hourswithin an oxygen atmosphere. In this step, an oxide film 99 containingnickel element in high concentration is formed. In the same time, theconcentration of nickel element within the silicon film 98 may bereduced relatively. Here, the thermal oxide film 99 is formed in athickness of 200 angstrom. The thermal oxide film contains the nickelelement gettered by the action of chlorine in high concentration.Further, because the thermal oxide film 99 is formed, the thickness ofthe crystal silicon film 98 is reduced to about 1900 angstrom.

Next, the thermal oxide film 99 containing nickel element in highconcentration is eliminated. In the crystal silicon film of this state,the nickel element has a distribution of concentration such that itexists in high concentration toward the surface of the crystal siliconfilm. Accordingly, it is useful to etch the surface of the crystalsilicon film to eliminate the region in which the nickel element existsin high concentration after eliminating the thermal oxide film 99. Thatis, the crystal silicon film in which the nickel element concentrationis reduced further may be obtained by etching the surface of the crystalsilicon film in which the nickel element exists in high concentration.

Next, patterning is implemented to form a pattern 100 formed of thelaterally grown region. Here, it is important to form the pattern 100such that no vertically grown region, amorphous region nor an edgeportion of the laterally grown region is included there, because theconcentration of nickel element is relatively high in the verticallygrown region and the edge portion of the laterally grown region and theamorphous region in which no crystal has grown has inferior electricalcharacteristics.

The concentration of nickel element which remains within the pattern 100made from the laterally grown region thus obtained may be reducedfurther as compared to the case shown in the sixteenth embodiment. Thisis caused by the fact that the concentration of the metal elementcontained within the laterally grown region is low originally. Inconcrete, the concentration of nickel element within the pattern 100made from the laterally grown region may be readily reduced to the orderof 10¹⁷ cm⁻³ or less.

When a thin film transistor is formed by utilizing the laterally grownregion, a semiconductor device having a higher mobility may be obtainedas compared to the case when the vertically grown region as shown in thesixteenth embodiment (crystal grows vertically on the whole surface inthe case of the sixteenth embodiment) is utilized. It is noted that itis useful to implement the etching process further after forming thepattern shown in FIG. 26E to eliminate the nickel element existing onthe surface of the pattern.

On the other hand, it is not effective to implement thermal oxidationwithin the oxidizing atmosphere containing halogen element afterpatterning the crystal silicon film in a shape of island and toeliminate the thermal oxide film. It is because in this arrangement, theetching advances so as to scoop out even the under side of the crystalsilicon film formed in an island shape because the etching of theunderlying film advances when the thermal oxide film is eliminated,though the gettering effect is certainly obtained by the thermal oxidefilm.

Such a condition may cause breaking of wires and defective operation ofelements later. Next, a thermal oxide film 101 is formed after thusforming the pattern 100. This thermal oxide film 101 is what becomes apart of a gate insulating film later in constructing a thin filmtransistor.

[22-nd Embodiment]

A 22-nd embodiment relates to a case of fabricating a thin filmtransistor disposed in a pixel region of an active matrix type liquidcrystal display or an active matrix type EL display by utilizing theinventive crystal silicon film. FIGS. 27A through 27E show thefabrication process according to the 22-nd embodiment.

At first, the crystal silicon film is formed on the glass substratethrough the process shown in the sixteenth or the 21-st embodiment. Alsoa thin film transistor is fabricated in the same manner. While the caseof using the crystal silicon film obtained by the arrangement shown inthe sixteenth embodiment will be described below, the same applies tothe case of using the crystal silicon film obtained by the arrangementshown in the 21-st embodiment. The state shown in FIG. 27A is obtainedby patterning the crystal silicon film. In FIG. 27A, the referencenumeral (103) denotes a glass substrate, (104) an underlying film, and(105) an active layer formed of the crystal silicon film.

After obtaining the state shown in FIG. 27A, a plasma treatment isimplemented within a reduced pressure atmosphere in which oxygen andhydrogen are mixed. The plasma is generated by high-frequency discharge.Organic substances existing on the surface of the exposed active layer105 may be removed by the above-mentioned plasma treatment.Specifically, the organic substances adsorbing on the surface of theactive layer are oxidized by oxygen plasma and the oxidized organicsubstances are reduced and vaporized by hydrogen plasma. Thus theorganic substances existing on the surface of the exposed active layer105 are removed. The removal of the organic substances is very effectivein suppressing fixed charge from existing on the surface of the activelayer 105.

That is, because the fixed charge caused by the existence of organicsubstances hampers the operation of the device and renders thecharacteristics thereof instable, it is very useful to remove it. Afterremoving the organic substances, thermal oxidation is implemented withinan oxygen atmosphere at 640° C. to form a thermal oxide film 102 of 100angstrom thick. This thermal oxide film has a high interfacialcharacteristic with a semiconductor layer and composes a part of a gateinsulating film later. Thus, the state shown in FIG. 27A is obtained.

After that, a silicon oxynitride film 106 which composes the gateinsulating film is formed in a thickness of 1000 angstrom. While thefilm may be formed by using plasma CVD using mixed gas of silane and N₂Oor plasma CVD using mixed gas of TEOS and N₂O, the latter is used in thepresent embodiment. The silicon oxynitride film 106 functions as thegate insulating film together with the thermal oxide film 102.

It is also effective to contain halogen element within the siliconoxynitride film. That is, it is possible to prevent the function of thegate insulating film as an insulating film from dropping by theinfluence of the nickel element (or another metal element which promotescrystallization of silicon) existing within the active layer by fixingthe nickel element by the action of the halogen element.

It is significant to use the silicon oxynitride film in that metalelement hardly infiltrates to the gate insulating film from its densefilm quality. If metal element infiltrates into the gate insulatingfilm, its function as an insulating film drops, thus causing instabilityand dispersion of characteristics of the thin film transistor. It isnoted that a silicon oxide film which is normally used may be also usedfor the gate insulating film.

After forming the silicon oxynitride film 106 which functions as thegate insulating film, an aluminum film (not shown) which functions as agate electrode later is formed by sputtering. 0.2 weight % of scandiumis included within the aluminum film to suppress hillock and whiskerfrom being produced in the later process. The hillock and whisker meanthat abnormal growth of aluminum occurs by heating, thus forming needleor prickle-like projections.

After forming the aluminum film, a dense anodic oxide film not shown isformed. The anodic oxide film is formed by using ethylene glycolsolution containing 3 weight % of tartaric acid as electrolyte. That is,the anodic oxide film having the dense film quality is formed on thesurface of the aluminum film by setting the aluminum film as the anodeand platinum as the cathode and by anodizing within this electrolyte.The thickness of the anodic oxide film not shown having the dense filmquality is around 100 angstrom. This anodic oxide film plays a role ofenhancing the adhesiveness with a resist mask to be formed later. It isnoted that the thickness of the anodic oxide film may be controlled byadjusting voltage applied during the anodization.

Next, the resist mask 108 is formed and the aluminum film is patternedso as to have a pattern 107. The state shown in FIG. 27B is thusobtained. Here, another anodization is implemented. In this case, 3weight % of oxalate aqueous solution is used as electrolyte. A porousanodic oxide film 110 is formed by anodizing within this electrolyte bysetting the aluminum pattern 107 as the anode.

In this step, the anodic oxide film 110 is formed selectively on thesides of the aluminum pattern because the resist mask 108 having thehigh adhesiveness exists thereabove. The anodic oxide film may be grownup to several μm thick. The thickness is 6000 angstrom in the presentembodiment. It is noted that the range of growth may be controlled byadjusting an anodizing time. Next, the resist mask 108 is removed.

Then, a dense anodic oxide film is formed again. That is, theanodization is implemented again by using the ethylene glycol solutioncontaining 3 weight % of tartaric acid as electrolyte. Then, an anodicoxide film 111 having a dense film quality is formed because theelectrolyte infiltrates into the porous anodic oxide film 110. Thisdense anodic oxide film 111 is 1000 angstrom thick. The thickness iscontrolled by adjusting applied voltage.

Here, the exposed silicon oxynitride film 106 and the thermal oxide film102 are etched by utilizing dry etching. Then, the porous anodic oxidefilm 110 is eliminated by using mixed acid in which acetic acid, nitricacid and phosphoric acid are mixed. Thus, the state shown in FIG. 27D isobtained. After obtaining the state shown in FIG. 27D, impurity ions areinjected. Here, P (phosphorus) ions are injected by plasma doping inorder to fabricate an N-channel type thin film transistor.

In this step, heavily doped regions 113 and 117 and lightly dopedregions 114 and 116 are formed because part of the remaining siliconoxide film 112 functions as a semi-permeable mask, thus blocking part ofthe injected ions. Next, laser light or intense light is irradiated toactivate the regions into which the impurity ions have been injected.Laser light is used in the present embodiment. Thus, a source region113, a channel forming region 115, a drain region 117 and lowconcentration impurity regions 114 and 116 are formed in a manner ofself-alignment.

The one designated by the reference numeral 116 here is the regioncalled the LDD (lightly doped drain). It is noted that when the denseanodic oxide film 111 is formed as thick as 2000 angstrom or more, anoffset gate region may be formed on the outside of the channel formingregion 115 by its thickness. Although the offset gate region is formedalso in the present embodiment, it is not shown in the figures becauseits size is small, its contribution due to the existence thereof issmall and the figures might otherwise become complicated.

Next, a silicon oxide film or a silicon nitride film or their laminatedfilm is formed as an interlayer insulating film 118. The silicon nitridefilm is used in the present embodiment. The interlayer insulating filmmay be constructed by forming a layer made from a resin material on thesilicon oxide film or the silicon nitride film. Next, contact holes arecreated to form a source electrode 119 and a drain electrode 120. Thus,the thin film transistor shown in FIG. 27E is completed.

[23-rd Embodiment]

A 23-rd embodiment is related to a method for forming the gateinsulating film 106 in the arrangement shown in the 22-nd embodiment.Thermal oxidation may be used as a method for forming the gateinsulating film when a quartz substrate or a glass substrate having ahigh heat resistance is used as the substrate. The thermal oxidationallows the film quality to be densified and is useful in obtaining athin film transistor having stable characteristics. That is, because anoxide film formed by the thermal oxidation is dense as an insulatingfilm and movable electric charge existing therein can be reduced, it isone of the most suitable films as a gate insulating film.

In the present embodiment, a heat treatment is implemented in anoxidizing atmosphere at 950° C. in forming the thermal oxide film. Atthis time, it is effective to mix HCl or the like into the oxidizingatmosphere because, thereby, metal element existing in the active layermay be fixed in the same time with the formation of the thermal oxidefilm. It is also effective to mix N₂O gas into the oxidizing atmosphereto form a thermal oxide film containing nitrogen component. Here, it isalso possible to obtain a silicon oxynitride film by the thermaloxidation if the mixed ratio of N₂O gas is optimized. It is noted thatthe thermal oxide film 102 needs not be formed in the presentembodiment.

[24-th Embodiment]

A 24-th embodiment relates to a case of fabricating a thin filmtransistor through a process different from that shown in the 22-nd and23-rd embodiments (FIG. 27). FIGS. 28A through 28E show the fabricationprocess according to the present embodiment. At first, the crystalsilicon film is formed on the glass substrate through the process shownin the sixteenth or seventeenth embodiment. It is then patterned, thusobtaining the state shown in FIG. 28A.

After obtaining the state shown in FIG. 28A, a plasma treatment isimplemented within a reduced pressure atmosphere in which oxygen andhydrogen are mixed. In the state shown in FIG. 28A, the referencenumeral (122) denotes a glass substrate, (123) an underlying film, (124)an active layer formed of the crystal silicon film and (121) a thermaloxide film formed again after eliminating the thermal oxide film forgettering.

After obtaining the state shown in FIG. 28A, a silicon oxynitride film125 which composes a gate insulating film is formed in a thickness of1000 angstrom. While the film may be formed by using plasma CVD usingmixed gas of oxygen, silane and N₂O or plasma CVD using mixed gas ofTEOS and N₂O, the former is used in the present embodiment. The siliconoxynitride film 125 composes the gate insulating film together with thethermal oxide film 121. It is noted that a silicon oxide film may beused beside the silicon oxynitride film.

After forming the silicon oxynitride film 125 which functions as thegate insulating film, an aluminum film not shown which functions as agate electrode later is formed by sputtering. 0.2 weight % of scandiumis included within the aluminum film. After forming the aluminum film, adense anodic oxide film not shown is formed. The anodic oxide film isformed by using ethylene glycol solution containing 3 weight % oftartaric acid as electrolyte. That is, the anodic oxide film having thedense film quality is formed on the surface of the aluminum film bysetting the aluminum film as the anode and platinum as the cathode andby anodizing within this electrolyte.

The thickness of the anodic oxide film not shown having the dense filmquality is around 100 angstrom. This anodic oxide film plays a role ofenhancing the adhesiveness with a resist mask to be formed later. It isnoted that the thickness of the anodic oxide film may be controlled byadjusting voltage applied during the anodization. Next, the resist mask126 is formed and the aluminum film is patterned so as to have a pattern127.

Here, another anodization is implemented. In this case, 3 weight % ofoxalate aqueous solution is used as electrolyte. A porous anodic oxidefilm 128 is formed by anodizing within this electrolyte by setting thealuminum pattern 127 as the anode. In this step, the anodic oxide film128 is formed selectively on the sides of the aluminum pattern becausethe resist mask 126 having the high adhesiveness exists thereabove.

The anodic oxide film may be grown up to several μm thick. The thicknessis 6000 angstrom in the present embodiment. It is noted that the rangeof growth may be controlled by adjusting an anodizing time. Next, afterremoving the resist mask 126, a dense anodic oxide film is formed again.That is, the anodization is implemented again by using the ethyleneglycol solution containing 3 weight % of tartaric acid as electrolyte.Then, an anodic oxide film 129 having a dense film quality is formedbecause the electrolyte infiltrates into the porous anodic oxide film128.

Here, the initial injection of impurity ions is implemented. It is notedthat this step may be implemented after removing the resist mask 126. Asource region 130 and a drain region 132 are formed by injecting theimpurity ions. No impurity ion is injected to a region 131. Then, theporous anodic oxide film 128 is eliminated by using mixed acid in whichacetic acid, nitric acid and phosphoric acid are mixed. Thus, the stateshown in FIG. 28D is obtained.

After obtaining the state shown in FIG. 28D, impurity ions are injectedagain. The impurity ions are injected under the doping condition lighterthan that of the first injection. In this step, lightly doped region 133and 134 are formed and a region 135 turns out to be a channel formingregion. Then, intense light is irradiated by using an infrared lamp toactivate the regions into which the impurity ions have been injected. Itis noted that laser light may be used instead of the intense light.Thus, the source region 130, the channel forming region 135, the drainregion 132 and low concentration impurity regions 133 and 134 are formedin a manner of self-alignment.

Here, the one designated by the reference numeral 134 is the regioncalled the LDD (lightly doped drain). Next, a silicon oxide film or asilicon nitride film or their laminated film is formed as an interlayerinsulating film 136. The silicon nitride film is used in the presentembodiment. The interlayer insulating film may be constructed by forminga layer made from a resin material on the silicon oxide film or thesilicon nitride film. After that, contact holes are created to form asource electrode 137 and a drain electrode 138. Thus, the thin filmtransistor shown in FIG. 28E is completed.

[25-th Embodiment]

A 25-th embodiment relates to a case when an N-channel type thin filmtransistor and a P-channel type thin film transistor are formed in acomplementary manner. FIGS. 29A through 29F are diagrams showing afabrication process according to the present embodiment. The formationshown in the present embodiment may be utilized for various thin filmintegrated circuits integrated on an insulating surface as well as forperipheral driving circuits of an active matrix type liquid crystaldisplay for example.

At first, a silicon oxide film or a silicon oxynitride film is formed asan underlying film 141 on a glass substrate 140 as shown in FIG. 29A. Itis preferable to use the silicon oxynitride film and it is used in thepresent embodiment. Next, an amorphous silicon film not shown is formedby the plasma CVD or low pressure thermal CVD. The low pressure thermalCVD is adopted in the present embodiment. Then, the amorphous siliconfilm is transformed into a crystal silicon film by the same method asshown in the sixteenth embodiment.

Next, a plasma treatment is implemented within an atmosphere in whichoxygen and hydrogen are mixed. Then, the obtained crystal silicon filmis patterned to obtain active layers 142 and 143. Thus, the state shownin FIG. 29A is obtained. It is noted that a heat treatment isimplemented at 650° C. for 10 hours within a nitrogen atmospherecontaining 3 volume % of HCl in the state shown in FIG. 29A in order tosuppress the influence of carriers moving the sides of the activelayers.

Because an OFF current characteristic becomes bad if a trap level existsdue to the existence of metal element on the sides of the active layers,it is effective to implement the above-mentioned treatment to lower thelevel on the sides of the active layers. Further, a thermal oxide film139 and a silicon oxynitride film 144 which compose a gate insulatingfilm are formed. When quartz is used as the substrate here, it isdesirable to compose the gate insulating film only by the thermal oxidefilm formed by using the above-mentioned thermal oxidation.

Next, an aluminum film not shown which composes a gate electrode lateris formed in a thickness of 4000 angstrom. Beside the aluminum film, ametal which can be anodized, such as tantalum, may be used. Afterforming the aluminum film, a very thin and dense anodic oxide film isformed on the surface thereof by the method described before. Next, aresist mask not shown is placed on the aluminum film to pattern thealuminum film. Then, anodization is implemented by setting the obtainedaluminum pattern as the anode to form porous anodic oxide films 147 and148.

The thickness of the porous anodic oxide films is 5000 angstrom. Then,another anodization is implemented under the condition of forming denseanodic oxide films 149 and 150. The thickness of the dense anodic oxidefilms 149 and 150 is 800 angstrom. Thus, the state shown in FIG. 29B isobtained. Then, the exposed silicon oxide film 144 and the thermal oxidefilm 139 are eliminated by dry etching, thus obtaining the state shownin FIG. 29C as a result.

After obtaining the state shown in FIG. 29C, the porous anodic oxidefilms 147 and 148 are eliminated by using mixed acid in which aceticacid, nitric acid and phosphoric acid are mixed. Thus, the state shownin FIG. 29D is obtained. Here, resist masks are disposed alternately toinject P (phosphorus) ions to the thin film transistor on the left sideand B (boron) ions to the thin film transistor on the right side. Byinjecting those impurity ions, a source region 153 and a drain region156 to which P ions are doped in high concentration, thus having N-type,are formed in a manner of self-alignment.

Further, a region 154 to which P ions are doped in low concentration,thus having weak N-type, as well as a channel forming region 155 areformed in the same time. The reason why the region 154 having the weakN-type is formed is because the remaining gate insulating film 151exists. That is, part of P ions transmitting through the gate insulatingfilm 151 is blocked by the gate insulating film 151.

By the same principle, a source region 160 and a drain region 157 havingstrong P-type are formed in a manner of self-alignment and a lowconcentration impurity region 159 is formed in the same time. Further, achannel forming region 158 is formed in the same time. It is noted thatwhen the thickness of the dense anodic oxide films 149 and 150 is asthick as 2000 angstrom, an offset gate region may be formed in contactwith the channel forming region by that thickness.

Its existence may be ignored in the case of the present embodimentbecause the dense anodic oxide films 149 and 150 are so thin as lessthan 1000 angstrom. Then, laser light is irradiated to anneal the regioninto which the impurity ions have been injected. Intense light may beused instead of the laser light. Then, a silicon nitride film 161 and asilicon oxide film 162 are formed as interlayer insulating films asshown in FIG. 29E. Their thickness is 1000 angstrom, respectively. It isnoted that the silicon oxide film 162 needs not be formed.

Here, the thin film transistor is covered by the silicon nitride film.The reliability of the thin film transistor may be enhanced by arrangingas described above because the silicon nitride film is dense and has anexcellent interfacial characteristic. Further, an interlayer insulatingfilm 163 made from a resin material is formed by means of spin coating.Here, the thickness of the interlayer insulating film 163 is 1 μm.

Then, contact holes are created to form a source electrode 164 and adrain electrode 165 of the N-channel type thin film transistor on theleft side. In the same time, a source electrode 166 and the drainelectrode 165 of the thin film transistor on the right side are formed.Here, the electrode 165 is disposed in common. Thus, the thin filmtransistor circuit having a CMOS structure constructed in acomplementary manner may be formed.

In the formation shown in the present embodiment, the thin filmtransistor is covered by the nitride film as well as the resin material.This formation allows to enhance the durability of the thin filmtransistor, so that movable ions nor moisture hardly infiltrate.Further, it allows to prevent capacitance from being generated betweenthe thin film transistor and wires when a multi-layered wire is formed.

[26-th Embodiment]

A 26-th embodiment relates to a case of forming a mono-crystal domain ora domain which can be substantially considered as a mono-crystal domainby irradiating laser light to the crystal silicon film obtained in thesixteenth or seventeenth embodiment.

At first, the crystal silicon film is obtained by utilizing the actionof nickel element as shown in the sixteenth embodiment. Next, laserlight is irradiated to the film to promote crystallization thereoffurther. KrF excimer laser is used as the laser light here. At thistime, the mono-crystal domain or the domain which can be substantiallyconsidered as a mono-crystal domain may be formed by using a heattreatment in the temperature range of more than 450° C. and byoptimizing the condition for irradiating laser light.

The film whose crystallization has been greatly promoted by such methodhas a domain which can be considered as a mono-crystal in which anelectron spin density measured by ESR is less than 3×10¹⁷ cm⁻³ and theconcentration of nickel element as the minimum value measured by SIMS isless than 3×10¹⁷ cm⁻³. Substantially, no grain boundary exists in thisdomain and high electrical characteristics equivalent to a mono-crystalsilicon wafer can be obtained.

Further, the domain which can be considered as a mono-crystal containshydrogen by less than 5 atom % to 1×10¹⁵ cm⁻³. This value is clarifiedby the measurement carried out by the SIMS (secondary ion massspectrometry). By fabricating a thin film transistor by utilizing suchmono-crystal or the domain which can be substantially considered as amono-crystal, a semiconductor device which is equivalent to a MOS typetransistor fabricated by using a mono-crystal wafer may be obtained.

[27-th Embodiment]

A 27-th embodiment relates to a case when nickel element is introduceddirectly to the surface of the underlying film in the process shown inthe sixteenth embodiment. In this case, the nickel element is held incontact on the lower surface of the amorphous silicon film. In thepresent embodiment, acetate nickel aqueous solution is applied afterforming the underlying film to introduce the nickel element such thatthe nickel element (metal element) is held in contact on the surface ofthe underlying film. Then, a thin film transistor similar to one shownin FIG. 25E is completed by implementing the other process in the samemanner with the case of the sixteenth embodiment. Beside the method ofusing the solution, sputtering, CVD or adsorption may be used as themethod for introducing nickel element.

[28-th Embodiment]

In a 28-th embodiment, an arrangement for obtaining a crystal siliconfilm on a glass substrate by utilizing nickel element will be shown. Inthe present embodiment, the crystal silicon film having a highcrystallinity is obtained by the action of nickel element at first.Then, laser light is irradiated to enhance the crystallinity of the filmand to diffuse the nickel element which is concentrated locally withinthe film, i.e. to extinguish blocks of nickel.

Next, a thermal oxide film containing a halogen element is formed on thecrystal silicon film by thermal oxidation. At this time, the nickelelement remaining in the crystal silicon film is gettered to the thermaloxide film by the action of oxygen and halogen element. At this time,because the nickel element is dispersed therein by the irradiation oflaser light, the gettering proceeds effectively. Then, the thermal oxidefilm containing the nickel element in high concentration as a result ofthe gettering is eliminated. Thereby, the crystal silicon film which hasthe high crystallinity and in which the concentration of nickel elementis low is obtained on the glass substrate.

FIGS. 30A through 30E are diagrams showing the fabrication processaccording to present embodiment. At first, a silicon oxynitride film 168is formed as an underlying film in a thickness of 3000 angstrom on theglass substrate 167 of Corning 1737 (distortion point: 667° C.). Thesilicon oxynitride film is formed by using the plasma CVD using silane,N₂O gas and oxygen as original gases in the present embodiment. TEOS gasand N₂O gas may be also used as the original gases.

The silicon oxynitride film has a function of suppressing the diffusionof impurities from the glass substrate in the later steps (seeing fromthe level of fabrication of a semiconductor, the glass substratecontains a large amount of impurities). It is noted that although thesilicon nitride film is the most suitable to obtain the function forsuppressing the diffusion of the impurities in maximum, the siliconnitride film is not practical because it is peeled off from the glasssubstrate due to the influence of stress. A silicon oxide film may bealso used as the underlying film.

It is also important to increase the hardness of the underlying film 168as much as possible. It is concluded from the fact that the harder thehardness of the underlying film (i.e. the smaller the etching ratethereof), the higher the reliability is in an endurance test of the thinfilm transistor obtained in the end. Although the reason thereof isunknown in detail, it is assumed to be caused by the effect of blockingthe impurities from the glass substrate in the fabrication process ofthe thin film transistor.

It is also effective to contain a small amount of halogen elementtypified by chlorine in the underlying film 168. Thereby, the metalelement which promotes crystallization of silicon which exists withinthe semiconductor layer may be gettered by the halogen element in thelater step. It is also effective to add a hydrogen plasma treatmentafter forming the underlying film. It is also effective to implement aplasma treatment in an atmosphere in which oxygen and hydrogen aremixed. These treatments are effective in eliminating carbon componentwhich is adsorbed on the surface of the underlying film and in enhancingthe characteristic of interface with a semiconductor film formed later.

Next, an amorphous silicon film 169, which turns out to be a crystalsilicon film later, is formed in a thickness of 500 angstrom by the lowpressure thermal CVD. The reason why the low pressure thermal CVD isused is because thereby, the quality of the crystal silicon filmobtained later is better, i.e. the film quality is denser in concrete.Beside the low pressure thermal CVD, the plasma CVD or the like may beused. The amorphous silicon film fabricated here is desirable to have5×10¹⁷ cm⁻³ to 2×10¹⁹ cm⁻³ of concentration of oxygen within the film.It is because oxygen plays an important role in the later step ofgettering the metal element which promotes crystallization of silicon.However, it must be careful here because the crystallization of theamorphous silicon film is hampered if the oxygen concentration is higherthan the above-mentioned range of concentration.

The concentration of other impurities such as those of nitrogen andcarbon is preferred to be low to the utmost. In concrete, theconcentration must be below 2×10¹⁹ cm⁻³. The upper limit of thethickness of the amorphous silicon film is about 2000 angstrom. It isbecause a thick film is disadvantageous in obtaining the effect of theirradiation of laser light implemented later. That is, the most of laserlight irradiated to a silicon film is absorbed by the surface of thefilm. The thickness of the amorphous silicon film must be thicker than athickness which is required in the end as described later. The lowerlimit of the thickness of the amorphous silicon film 169 is practicallyabout 200 angstrom, though it depends on a method for forming the film.

Next, nickel element is introduced to the amorphous silicon film 169 tocrystallize it. Here, the nickel element is introduced by applyingnickel acetate solution containing 10 ppm (weight conversion) of nickelon the surface of the amorphous silicon film 169. Beside the method ofusing the above-mentioned solution, sputtering, CVD, plasma treatment oradsorption may be used as the method for introducing the nickel element.

Among them, the method of using the solution is useful in that it issimple and that the concentration of the metal element may be readilyadjusted. The nickel acetate solution is applied as described above toform a water film (liquid film) 170 of the nickel acetate solution asshown in FIG. 30A. After obtaining this state, extra solution is blownout by using a spin coater not shown. Thus, the nickel element is heldin contact on the surface of the amorphous silicon film 169.

It is noted that it is preferable to use nickel sulfate solution,instead of the nickel acetate, if the remained impurities in the laterheating process is taken into consideration. It is because the nickelacetate aqueous solution contains carbon and it might be carbonized inthe later heating process, thus remaining within the film. An amount ofthe nickel element to be introduced may be controlled by adjusting theconcentration of nickel salt within the solution.

Next, a heat treatment is implemented in the temperature range from 550°C. to 650° C. in the state shown in FIG. 30B to crystallize theamorphous silicon film 169 and to obtain a crystal silicon film 171.This heat treatment is implemented in a reducing atmosphere. Thetemperature of this heat treatment is preferable to be a temperaturebelow the distortion point of the glass substrate. Because thedistortion point of the Corning 1737 glass substrate is 667° C., it ispreferable to set the upper limit of the heating temperature here atabout 650° C., leaving some margin. Here, the heat treatment isimplemented for four hours at 620° C. within a nitrogen atmospherecontaining 3 volume % of hydrogen.

The reason why the reducing atmosphere is adopted in the crystallizationstep in a way of the heat treatment is to prevent oxides from beingcreated in the step of the heat treatment and more concretely, tosuppress nickel from reacting with oxygen and NiOx from being created onthe surface of the film or within the film. Oxygen couples with nickeland contributes a lot in gettering nickel in the later gettering step.

However, it has been found that if oxygen couples with nickel in theabove-mentioned stage of the crystallization, it hampers thecrystallization. Accordingly, it is important to suppress the oxidesfrom being created to the utmost in the crystallization step in a way ofheating. The concentration of oxygen within the atmosphere forimplementing the heat treatment for the crystallization has to be in anorder of ppm, or preferably, less than 1 ppm.

Inert gases such as argon, beside nitrogen, or their mixed gas may beused as the gas which occupies the most of the atmosphere forimplementing the heat treatment for the crystallization. After thecrystallization step by the heat treatment described above, nickelelement remains in certain blocks. It has been confirmed by anobservation through a TEM (Transmission type Electron Microscope). Whilethe reason why nickel exists in certain blocks has not been clarifiedyet, it is considered to be related with some crystallization mechanism.

Next, laser light is irradiated as shown in FIG. 30C. Here, KrF excimerlaser (wavelength: 248 nm) is used. The irradiation is implemented byscanning the laser beam whose shape is linear. The irradiation of thelaser light allows the nickel element which has been concentratedlocally as a result of the crystallization by means of theabove-mentioned heat treatment to be distributed within the film 171 insome degree. That is, it allows to extinguish the blocks of nickelelement and to distribute the nickel element.

Another heat treatment is implemented in a step in FIG. 30D to form athermal oxide film for gettering the nickel element. This heat treatmentis implemented within an atmosphere containing halogen element. Inconcrete, this heat treatment is implemented within the oxygenatmosphere containing 5 volume % of HCl. This step is carried out toeliminate the nickel element (or other metal element which promotescrystallization of silicon) which has been introduced intentionally forthe crystallization in the initial stage from the crystal silicon film171.

This heat treatment is implemented at a temperature higher than that ofthe heat treatment implemented for the crystallization described above.It is an important condition for effectively implementing the getteringof nickel element. It is noted that although this heat treatment may beimplemented in the same or less temperature in the heat treatmentimplemented for the crystallization, it is less effective.

This heat treatment is implemented in the temperature range from 600° C.to 750° C. upon meeting the above-mentioned condition. The effect forgettering nickel element may be obtained remarkably in this step if thetemperature is higher than 600° C. In this step, the nickel elementwhich has been distributed by the irradiation of laser light describedabove is gettered effectively to the oxide film. The upper limit of theheating temperature is limited by the distortion point of the glasssubstrate to be used. It must be careful not to implement the heattreatment in a temperature above the distortion point of the glasssubstrate to be used because, otherwise, it is deformed.

It is preferable to mix HCl with a ratio of 0.5% to 10% (volume %) tooxygen. It must be careful not to mix above this concentration because,otherwise, the surface of the film becomes rough with the same or moredegree of irregularity with the thickness of the film. A thermal oxidefilm 172 containing chlorine as shown in FIG. 30D is formed byimplementing the heat treatment under such condition. Here, the thermaloxide film 172 is formed in a thickness of 100 angstrom by implementingthe heat treatment for 12 hours.

Because the thermal oxide film 172 is formed, the thickness of thecrystal silicon film 169 reaches to about 450 angstrom. When the heatingtemperature is 600° C. to 750° C. in the heat treatment, the treatmenttime (heating time) is set at 10 hours to 48 hours or typically at 24hours. This treatment time may be set adequately depending on thethickness of the oxide film to be obtained as a matter of course. Inthis step, nickel element is gettered out of the silicon film by theaction of the halogen element. Here, the nickel element is gettered tothe thermal oxide film 172 to be formed by the action of chlorine.

In the gettering, oxygen is also involved. That is, in this gettering,oxygen existing within the crystal silicon film plays an important role.That is, the gettering of nickel element proceeds effectively becausethe gettering effect caused by chlorine acts on nickel oxide formed whenoxygen couples with nickel. If the concentration of oxygen is too much,it becomes the factor of hampering the crystallization of the amorphoussilicon film 169 in the crystallization step shown in FIG. 30B asdescribed above. However, the existence thereof plays an important rolein the process of gettering nickel as described above. Accordingly, itis important to control the concentration of oxygen existing within theamorphous silicon film, the starting film.

Here, Cl has been selected as the halogen element and the case of usingHCl has been shown as a method for introducing it. Beside HCl, one typeor a plurality of types of mixed gas selected from HF, HBr, Cl₂, F₂, Br₂may be used. Beside them, halogen hydride may be used in general. It ispreferable to set the content (volume content) of those gases within theatmosphere to 0.25 to 5% if it is HF, 1 to 15% if it is HBr, 0.25 to 5%if it is Cl₂, 0.125 to 2.5% if it is F₂ and 0.5 to 10% if it is Br₂.

If the concentration is below the above-mentioned range, no significanteffect can be obtained. Further, if the concentration exceeds theabove-mentioned range, the surface of the crystal silicon film isroughened. Through this step, the concentration of nickel element may bereduced to 1/10 from the initial stage. It means that the nickel elementmay be reduced to 1/10 as compared to the case when no gettering isconducted by the halogen element. This effect may be obtained in thesame manner even when another metal element is used. Because the nickelelement is gettered to the oxide film to be formed in the aforementionedstep, naturally the nickel concentration within the oxide film becomeshigh as compared to other regions.

Further, it has been observed that the concentration of nickel elementis apt to be high near the interface between the crystal silicon film171 and the thermal oxide film 172. It is considered to happen becausethe region where the gettering mainly takes place is on the side of theoxide film near the interface between the crystal silicon film and theoxide film. The gettering proceeding near the interface is considered tobe caused by the existence of stress and defects near the interface.Next, the thermal oxide film 172 containing nickel in high concentrationis eliminated. While the thermal oxide film 172 may be eliminated bymeans of dry etching or wet etching using buffer hydrofluoric acid orother hydrofluorite (hydrofluoric) etchant, it is implemented by the wetetching by using the buffer hydrofluoric acid in the present embodiment.

Thus, a crystal silicon film 173 in which the concentration of nickelhas been reduced is obtained as shown in FIG. 30E. Because nickelelement is contained near the surface of the obtained crystal siliconfilm 173 relatively in high concentration, it is effective to advancethe etching of the above-mentioned oxide film 172 to over-etch, more orless, the surface of the crystal silicon film 173.

It is also effective to irradiate laser light again after eliminatingthe thermal oxide film 172 to promote the crystallization of theobtained crystal silicon film 173 further. That is, it is effective toirradiate the laser light again after gettering nickel element. KrFexcimer laser (wavelength: 248 nm) is used as the laser light to be usedin the present embodiment. However, XeCl excimer laser (wavelength: 308nm) and other types of lasers may be also used. Further, it is possibleto arrange so as to irradiate ultraviolet ray or infrared ray, insteadof the laser light.

[29-th Embodiment]

A 29-th embodiment relates to a case when Cu is used as the metalelement which promotes crystallization of silicon in the arrangementshown in the 28-th embodiment. In this case, while solutions such ascupric acetate [Cu(CH₃COO)₂] and cupric chloride (CuCl₂2H₂O) may be usedas the solution for introducing Cu, the former is used in the presentembodiment.

[30-th Embodiment]

A 30-th embodiment relates to a case of growing crystal in the formdifferent from that in the 28-th embodiment. That is, the presentembodiment relates to a method of growing the crystal in a directionparallel to the substrate, i.e. a method called lateral growth, byutilizing the metal element which promotes crystallization of silicon.FIGS. 31A through 31E show the fabrication process according to the30-th embodiment.

At first, a silicon oxynitride film is formed as an underlying film 175in a thickness of 3000 angstrom on the Corning 1737 glass substrate 174.It is noted that a quartz substrate may be used instead of the glasssubstrate. Next, an amorphous silicon film 176 which is the startingfilm of a crystal silicon film is formed in a thickness of 600 angstromby low pressure thermal CVD. The thickness of the amorphous silicon filmis preferable to be less than 2000 angstrom as described before. It isnoted that plasma CVD may be also used instead of the low pressurethermal CVD.

Next, a silicon oxide film not shown is formed in a thickness of 1500angstrom and is patterned to form a mask 177. An opening is created onthe mask in a region 178. The amorphous silicon film 176 is exposed atthe region where the opening 178 is created. The opening 178 has a thinand long rectangular shape in the longitudinal direction from the depthto the front side of the figure. Preferably, the width of the opening178 is 20 μm or more. The length thereof in the longitudinal directionmay be determined arbitrarily.

Then, the nickel acetate aqueous solution containing 10 ppm of nickelelement in terms of weight is applied in the same manner with the 28-thembodiment and the extra solution is removed by implementing spin dryingby using a spinner not shown. Thus, the solution is held in contact onthe exposed surface of the amorphous silicon film 176 as indicated by adot line 179 in FIG. 31A.

Next, a heat treatment is implemented at 640° C. for four hours in anitrogen atmosphere containing 3 volume % of hydrogen and in whichoxygen is minimized. Then, crystal grows in the direction parallel tothe substrate 174 as indicated by the reference numeral 180 in FIG. 31B.This crystal growth advances from the region of the opening 178 to whichnickel element has been introduced to the surrounding part. This crystalgrowth in the direction parallel to the substrate will be referred to aslateral growth throughout the present specification.

It is possible to advance this lateral growth across more than 100 μmunder the conditions shown in the present embodiment. Then, a crystalsilicon film 181 having the region in which the crystal has thus grownlaterally is obtained. It is noted that crystal growth in the verticaldirection called vertical growth advances from the surface of thesilicon film to the underlying interface in the region where the opening178 is formed.

Then, the mask 177 made from the silicon oxide film for selectivelyintroducing nickel element is eliminated. Thus, the state shown in FIG.31C is obtained. In this state, the vertically grown region, thelaterally grown region and a region in which no crystal has grown(having amorphous state) exist within the silicon film 181. In thisstate, nickel element is unevenly distributed within the film.Specifically, the nickel element exists relatively in high concentrationin the region where the opening 178 is created and in the edge portionin the direction of the crystal growth indicated by the referencenumeral 180.

After obtaining the state shown in FIG. 31C, laser light is irradiated.Here, KrF excimer laser is irradiated in the same manner with the 28-thembodiment. Thereby, the nickel element unevenly distributed may bediffused, allowing to obtain a state in which gettering may beimplemented readily in the later gettering step. After irradiating thelaser light, a heat treatment is implemented at 650° C. for 12 hourswithin an oxygen atmosphere containing 3 volume % of HCl. In this step,an oxide film 182 containing nickel element in high concentration isformed. In the same time, the concentration of nickel element within thesilicon film 181 may be reduced relatively.

Here, the thermal oxide film 182 is formed in a thickness of 100angstrom. The thermal oxide film contains the nickel element gettered bythe action of chlorine in high concentration. Further, because thethermal oxide film 182 is formed, the thickness of the crystal siliconfilm 181 is reduced to about 500 angstrom. Next, the thermal oxide film182 containing nickel element in high concentration is eliminated.

In the crystal silicon film of this state, the nickel element has adistribution of concentration such that it exists in high concentrationtoward the surface of the crystal silicon film. This is caused by thefact that the nickel element has been gettered to the thermal oxide filmwhen the thermal oxide film 182 has been formed. Accordingly, it isuseful to etch the surface of the crystal silicon film to eliminate theregion in which the nickel element exists in high concentration aftereliminating the thermal oxide film 182. That is, the crystal siliconfilm in which the nickel element concentration is reduced further may beobtained by etching the surface of the crystal silicon film in which thenickel element exists in high concentration. However, in this case, thethickness of the silicon film finally obtained needs to be taken intoconsideration.

Next, patterning is implemented to form a pattern 183 formed of thelaterally grown region. The concentration of nickel element whichremains within the pattern 183 made from the laterally grown region thusobtained may be reduced further as compared to the case shown in the28-th embodiment. This is caused by the fact that the concentration ofthe metal element contained within the laterally grown region is loworiginally. In concrete, the concentration of nickel element within thepattern 183 made from the laterally grown region may be readily reducedto the order of 10¹⁷ cm⁻³ or less.

When a thin film transistor is formed by utilizing the laterally grownregion, a semiconductor device having a higher mobility may be obtainedas compared to the case when the vertically grown region as shown in the28-th embodiment (crystal grows vertically on the whole surface in thecase of the 28-th embodiment) is utilized. It is noted that it is usefulto implement the etching process further after forming the pattern shownin FIG. 31E to eliminate the nickel element existing on the surface ofthe pattern.

Next, a thermal oxide film 184 is formed on the pattern 183. The thermaloxide film is formed in a thickness of 200 angstrom by implementing aheat treatment for 12 hours in an oxygen atmosphere at 650° C. It isnoted that this thermal oxide film becomes a part of a gate insulatingfilm later in constructing a thin film transistor. When the thin filmtransistor is fabricated after that, a silicon oxide film is formed byplasma CVD or the like over the thermal oxide film 184 to form the gateinsulating film.

[31-st Embodiment]

A 31-st embodiment relates to a case of fabricating a thin filmtransistor disposed in a pixel region of an active matrix type liquidcrystal display or an active matrix type EL display. FIGS. 32A through32E show the fabrication process according to the present embodiment. Atfirst, the crystal silicon film is formed on the glass substrate throughthe process shown in the 28-th or 30-th embodiment.

While the case of using the crystal silicon film obtained through theprocess shown in the 28-th embodiment will be described below, the sameapplies to the case of using the crystal silicon film obtained throughthe process shown in the 30-th embodiment. The state shown in FIG. 32Ais obtained by patterning the crystal silicon film obtained through theprocess in the 28-th embodiment. In FIG. 32A, the reference numeral(186) denotes a glass substrate, (187) an underlying film, and (188) anactive layer formed of the crystal silicon film. After obtaining thestate shown in FIG. 32A, a plasma treatment is implemented within areduced pressure atmosphere in which oxygen and hydrogen are mixed. Theplasma is generated by high-frequency discharge.

Organic substances existing on the surface of the exposed active layer188 may be removed by the plasma treatment. Specifically, the organicsubstances adsorbing on the surface of the active layer are oxidized byoxygen plasma and the oxidized organic substances are reduced andvaporized by hydrogen plasma. Thus the organic substances existing onthe surface of the exposed active layer 188 are removed. The removal ofthe organic substances is very effective in suppressing fixed chargefrom existing on the surface of the active layer 188. That is, becausethe fixed charge caused by the existence of organic substances hampersthe operation of the device and renders the characteristics thereofinstable, it is very useful to remove it.

After removing the organic substances, thermal oxidation is implementedwithin an oxygen atmosphere at 640° C. to form a thermal oxide film 185of 100 angstrom thick. This thermal oxide film has a high interfacialcharacteristic with a semiconductor layer and composes a part of a gateinsulating film later. Thus, the state shown in FIG. 32A is obtained.After that, a silicon oxynitride film 189 which composes the gateinsulating film is formed in a thickness of 1000 angstrom. In formingthe film, plasma CVD using mixed gas of silane and N₂O and oxygen isused. It is noted that plasma CVD using mixed gas of TEOS and N₂O may beused.

The silicon oxynitride film 189 functions as the gate insulating filmtogether with the thermal oxide film 185. It is also effective tocontain halogen element within the silicon oxynitride film. That is, itis possible to prevent the function of the gate insulating film as aninsulating film from dropping by the influence of the nickel element (oranother metal element which promotes crystallization of silicon)existing within the active layer by fixing the nickel element by theaction of the halogen element.

It is significant to use the silicon oxynitride film in that metalelement hardly infiltrates to the gate insulating film from its densefilm quality. If metal element infiltrates into the gate insulatingfilm, its function as an insulating film drops, thus causing instabilityand dispersion of characteristics of the thin film transistor. It isnoted that a silicon oxide film which is normally used may be also usedfor the gate insulating film.

After forming the silicon oxynitride film 189 which functions as thegate insulating film, an aluminum film not shown which functions as agate electrode later is formed by sputtering. 0.2 weight % of scandiumis included within the aluminum film to suppress hillock and whiskerfrom being produced in the later process. The hillock and whisker meanthat abnormal growth of aluminum occurs by heating, thus forming needleor prickle-like projections.

After forming the aluminum film, a dense anodic oxide film not shown isformed. The anodic oxide film is formed by using ethylene glycolsolution containing 3 weight % of tartaric acid as electrolyte. That is,the anodic oxide film having the dense film quality is formed on thesurface of the aluminum film by setting the aluminum film as the anodeand platinum as the cathode and by anodizing within this electrolyte.The thickness of the anodic oxide film not shown having the dense filmquality is around 100 angstrom. This anodic oxide film plays a role ofenhancing the adhesiveness with a resist mask to be formed later. It isnoted that the thickness of the anodic oxide film may be controlled byadjusting voltage applied during the anodization.

Next, the resist mask 191 is formed and the aluminum film is patternedso as to have a pattern 190. The state shown in FIG. 32B is thusobtained. Here, another anodization is implemented. In this case, 3weight % of oxalate aqueous solution is used as electrolyte. A porousanodic oxide film 193 is formed by anodizing within this electrolyte bysetting the aluminum pattern 190 as the anode.

In this step, the anodic oxide film 193 is formed selectively on thesides of the aluminum pattern because the resist mask 191 having thehigh adhesiveness exists thereabove. The anodic oxide film 193 may begrown up to several μm thick. The thickness is 6000 angstrom in thepresent embodiment. It is noted that the range of growth may becontrolled by adjusting an anodizing time.

Next, the resist mask 191 is removed. Then, a dense anodic oxide film isformed again. That is, the anodization is implemented again by using theethylene glycol solution containing 3 weight % of tartaric acid aselectrolyte. Then, an anodic oxide film 194 having a dense film qualityis formed because the electrolyte infiltrates into the porous anodicoxide film 193. This dense anodic oxide film 194 is 1000 angstrom thick.The thickness is controlled by adjusting applied voltage.

Here, the exposed silicon oxynitride film 189 and the thermal oxide film185 are etched by utilizing dry etching. Then, the porous anodic oxidefilm 193 is eliminated by using mixed acid in which acetic acid, nitricacid and phosphoric acid are mixed. Thus, the state shown in FIG. 32D isobtained. After obtaining the state shown in FIG. 32D, impurity ions areinjected.

Here, P (phosphorus) ions are injected by plasma doping in order tofabricate an N-channel type thin film transistor. In this step, heavilydoped regions 196 and 200 and lightly doped regions 197 and 199 areformed because part of the remaining silicon oxynitride film 195functions as a semi-permeable mask, thus blocking part of the injectedions.

Next, laser light or intense light is irradiated to activate the regionsinto which the impurity ions have been injected. The irradiation isimplemented using the intense light in the present embodiment. Thus, asource region 196, a channel forming region 198, a drain region 200 andlow concentration impurity regions 197 and 199 are formed in a manner ofself-alignment. The one designated by the reference numeral 199 here isthe region called the LDD (lightly doped region).

It is noted that when the dense anodic oxide film 194 is formed as thickas 2000 angstrom or more, an offset gate region may be formed on theoutside of the channel forming region 198 by its thickness. Although theoffset gate region is formed also in the present embodiment, it is notshown in the figures because its size is small, its contribution due tothe existence thereof is small and the figures might otherwise becomecomplicated.

Next, a silicon oxide film or a silicon nitride film or their laminatedfilm is formed as an interlayer insulating film 201. The silicon nitridefilm is used in the present embodiment. The interlayer insulating filmmay be constructed by forming a layer made from a resin material on thesilicon oxide film or the silicon nitride film. Then, contact holes arecreated to form a source electrode 202 and a drain electrode 203. Thus,the thin film transistor shown in FIG. 32E is completed.

[32-nd Embodiment]

A 32-nd embodiment is related to a method for forming the gateinsulating film 189 in the arrangement shown in the 31-st embodiment(FIG. 32). Thermal oxidation may be used as a method for forming thegate insulating film when a quartz substrate or a glass substrate havinga high heat resistance is used as the substrate. The thermal oxidationallows the film quality to be densified and is useful in obtaining athin film transistor having stable characteristics.

That is, because an oxide film formed by the thermal oxidation is denseas an insulating film and movable electric charge existing therein canbe reduced, it is one of the most suitable films as a gate insulatingfilm. In the present embodiment, a heat treatment is implemented in anoxidizing atmosphere at 950° C. A thin film transistor as shown in FIG.32E is completed by implementing other steps in the same manner as shownin the 31-st embodiment. At this time, it is effective to mix HCl or thelike into the oxidizing atmosphere because, thereby, metal elementexisting in the active layer may be fixed in the same time with theformation of the thermal oxide film. It is also effective to mix N₂O gasinto the oxidizing atmosphere to form a thermal oxide film containingnitrogen component. Here, it is also possible to obtain a siliconoxynitride film by the thermal oxidation if the mixed ratio of N₂O gasis optimized. It is noted that the thermal oxide film 185 needs not beformed specifically in the present embodiment.

[33-rd Embodiment]

A 33-rd embodiment relates to a case of fabricating a thin filmtransistor through a process different from that shown in the 31-stembodiment (FIG. 32). FIGS. 33A through 33E show the fabrication processaccording to the present embodiment. At first, the crystal silicon filmis formed on the glass substrate through the process shown in the 28-thor 30-th embodiment. It is then patterned, thus obtaining the stateshown in FIG. 33A. While the following process will be described basedon the process shown in the 30-th embodiment, the same applies also tothe process shown in the 28-th embodiment.

After obtaining the state shown in FIG. 33A, a plasma treatment isimplemented within a reduced pressure atmosphere in which oxygen andhydrogen are mixed. In the state shown in FIG. 33A, the referencenumeral (205) denotes a glass substrate, (206) an underlying film, (207)an active layer formed of the crystal silicon film and (204) a thermaloxide film formed again after eliminating the thermal oxide film forgettering. After obtaining the state shown in FIG. 33A, a siliconoxynitride film 208 which composes a gate insulating film is formed in athickness of 1000 angstrom. The film is formed by using plasma CVD usingmixed gas of oxygen, silane and N₂O. It is noted that plasma CVD usingmixed gas of TEOS and N₂O may be also used in forming the film.

The silicon oxynitride film 208 composes the gate insulating filmtogether with the thermal oxide film 204. It is noted that a siliconoxide film may be used beside the silicon oxynitride film. After formingthe silicon oxynitride film 208 which functions as the gate insulatingfilm, an aluminum film not shown which functions as a gate electrodelater is formed by sputtering. 0.2 weight % of scandium is includedwithin the aluminum film.

After forming the aluminum film, a dense anodic oxide film not shown isformed. The anodic oxide film is formed by using ethylene glycolsolution containing 3 weight % of tartaric acid as electrolyte. That is,the anodic oxide film having the dense film quality is formed on thesurface of the aluminum film by setting the aluminum film as the anodeand platinum as the cathode and by anodizing within this electrolyte.The thickness of the anodic oxide film not shown having the dense filmquality is around 100 angstrom. This anodic oxide film plays a role ofenhancing the adhesiveness with a resist mask to be formed later. It isnoted that the thickness of the anodic oxide film may be controlled byadjusting voltage applied during the anodization.

Next, the resist mask 209 is formed and the aluminum film is patternedso as to have a pattern 210. Here, another anodization is implemented.In this case, 3 weight % of oxalate aqueous solution is used aselectrolyte. A porous anodic oxide film 211 is formed by anodizingwithin this electrolyte by setting the aluminum pattern 210 as theanode.

In this step, the anodic oxide film 211 is formed selectively on thesides of the aluminum pattern because the resist mask 209 having thehigh adhesiveness exists thereabove. The anodic oxide film 211 may begrown up to several μm thick. The thickness is 6000 angstrom in thepresent embodiment. It is noted that the range of growth may becontrolled by adjusting an anodizing time.

Next, after removing the resist mask 209, a dense anodic oxide film isformed again. That is, the anodization is implemented again by using theethylene glycol solution containing 3 weight % of tartaric acid aselectrolyte. Then, an anodic oxide film 212 having a dense film qualityis formed because the electrolyte infiltrates into the porous anodicoxide film 211. Here, the initial injection of impurity ions isimplemented. This step may be implemented after removing the resist mask209. A source region 213 and a drain region 215 are formed by injectingthe impurity ions. It is noted that no impurity ion is injected to aregion 214.

Then, the porous anodic oxide film 211 is eliminated by using mixed acidin which acetic acid, nitric acid and phosphoric acid are mixed. Thus,the state shown in FIG. 33D is obtained. After obtaining the state shownin FIG. 33D, impurity ions are injected again. The impurity ions areinjected under the doping condition lighter than that of the firstinjection. In this step, lightly doped regions 216 and 217 are formedand a region 218 turns out to be a channel forming region.

Then, laser light or intense light is irradiated to activate the regionsinto which the impurity ions have been injected. The laser light is usedhere. Thus, the source region 213, the channel forming region 218, thedrain region 215 and low concentration impurity regions 216 and 217 areformed in a manner of self-alignment. Here, the one designated by thereference numeral 217 is the region called the LDD (lightly dopeddrain).

Next, while a silicon oxide film or a silicon nitride film or theirlaminated film is formed as an interlayer insulating film 219, thelaminated film of the silicon oxide film and the silicon nitride film isformed here. The interlayer insulating film may be also constructed byforming a layer made from a resin material on the silicon oxide film orthe silicon nitride film. After that, contact holes are created to forma source electrode 220 and a drain electrode 221. Thus, the thin filmtransistor shown in FIG. 33E is completed.

[34-th Embodiment]

A 34-th embodiment relates to a case when an N-channel type thin filmtransistor and a P-channel type thin film transistor are formed in acomplementary manner. The formation shown in the present embodiment maybe utilized for various thin film integrated circuits integrated on aninsulating surface as well as for peripheral driving circuits of anactive matrix type liquid crystal display for example. FIGS. 34A through34F are diagrams showing a fabrication process according to the presentembodiment.

At first, a silicon oxide film or a silicon oxynitride film is formed asan underlying film 224 on a glass substrate 223 as shown in FIG. 34A. Itis preferable to use the silicon oxynitride film, and it is used in thepresent embodiment. Next, an amorphous silicon film not shown is formedby the plasma CVD. It may be formed also by low pressure thermal CVD.Then, the amorphous silicon film is transformed into a crystal siliconfilm by the method shown in the 28-th embodiment.

Next, a plasma treatment is implemented within an atmosphere in whichoxygen and hydrogen are mixed. Then, the crystal silicon film thusobtained is patterned to obtain active layers 225 and 226. Thus, thestate shown in FIG. 34A is obtained. Further, a heat treatment isimplemented at 650° C. for 10 hours within a nitrogen atmospherecontaining 3 volume % of HCl in the state shown in FIG. 34A in order tosuppress the influence of carriers moving the sides of the activelayers.

Because an OFF current characteristic becomes bad if a trap level existsdue to the existence of metal element on the sides of the active layers,it is effective to implement the above-mentioned treatment to lower thelevel on the sides of the active layers. Further, a thermal oxide film222 and a silicon oxynitride film 227 which compose a gate insulatingfilm are formed. When quartz is used as the substrate here, it isdesirable to compose the gate insulating film only by the thermal oxidefilm formed by using the above-mentioned thermal oxidation.

Next, an aluminum film not shown which composes a gate electrode lateris formed in a thickness of 4000 angstrom. Beside the aluminum film, ametal which can be anodized, such as tantalum, may be used. Afterforming the aluminum film, a very thin and dense anodic oxide film isformed on the surface thereof by the method described before. Next, aresist mask not shown is placed on the aluminum film to pattern thealuminum film.

Then, anodization is implemented by setting the obtained aluminumpattern as the anode to form porous anodic oxide films 230 and 231. Thethickness of the porous anodic oxide films is 5000 angstrom. Then,another anodization is implemented under the condition of forming denseanodic oxide films 232 and 233. The thickness of the dense anodic oxidefilms 232 and 233 is 800 angstrom. Thus, the state shown in FIG. 34B isobtained.

Further, the exposed silicon oxynitride film 227 and the thermal oxidefilm 222 are eliminated by dry etching, thus obtaining the state shownin FIG. 34C as a result. After obtaining the state shown in FIG. 34C,the porous anodic oxide films 230 and 231 are eliminated by using mixedacid in which acetic acid, nitric acid and phosphoric acid are mixed.Thus, the state shown in FIG. 34D is obtained. Here, resist masks aredisposed alternately to inject P (phosphorus) ions to the thin filmtransistor on the left side and B (boron) ions to the thin filmtransistor on the right side.

By injecting those impurity ions, a source region 236 and a drain region239 to which P ions are doped in high concentration, thus having N-type,are formed in a manner of self-alignment. Further, a region 237 to whichP ions are doped in low concentration, thus having weak N-type, as wellas a channel forming region 238 are formed in the same time. The reasonwhy the region 237 having the weak N-type is formed is because theremaining gate insulating film 234 exists. That is, part of P ionstransmitting through the gate insulating film 234 is blocked by the gateinsulating film 234.

By the same principle, a source region 243 and a drain region 240 havingstrong P-type are formed in a manner of self-alignment and a lowconcentration impurity region 242 is formed in the same time. Further, achannel forming region 241 is formed in the same time. It is noted thatwhen the thickness of the dense anodic oxide films 232 and 233 is asthick as 2000 angstrom, an offset gate region may be formed in contactwith the channel forming region by that thickness.

Its existence may be ignored in the case of the present embodimentbecause the dense anodic oxide films 232 and 233 are so thin as lessthan 1000 angstrom. Then, laser light is irradiated to anneal the regioninto which the impurity ions have been injected. Intense light may bealso used instead of the laser light. Then, a silicon nitride film 244and a silicon oxide film 245 are formed as interlayer insulating filmsas shown in FIG. 34E. Their thickness is 1000 angstrom, respectively. Itis noted that the silicon oxide film 245 needs not be formed.

Here, the thin film transistor is covered by the silicon nitride film.The reliability of the thin film transistor may be enhanced by arrangingas described above because the silicon nitride film is dense and has anexcellent interfacial characteristic. Further, an interlayer insulatingfilm 246 made from a resin material is formed by means of spin coating.Here, the thickness of the interlayer insulating film 246 is 1 μm.

Then, contact holes are created to form a source electrode 247 and adrain electrode 248 of the N-channel type thin film transistor on theleft side. In the same time, a source electrode 249 and the drainelectrode 248 of the thin film transistor on the right side are formed(the electrode 248 is disposed in common) to complete the thin filmtransistor shown in FIG. 34F. Thus, the thin film transistor circuithaving a CMOS structure constructed in a complementary manner may beformed.

In the formation shown in the present embodiment, the thin filmtransistor is covered by the nitride film as well as the resin material.This formation allows to enhance the durability of the thin filmtransistor, so that movable ions nor moisture hardly infiltrate.Further, it allows to prevent capacitance from being generated betweenthe thin film transistor and wires when a multi-layered wire is formed.

[35-th Embodiment]

A 35-th embodiment relates to a case when nickel element is introduceddirectly to the surface of the underlying film in the process shown inthe 28-th embodiment (FIG. 30). In this case, the nickel element is heldin contact on the lower surface of the amorphous silicon film. In thepresent embodiment, the nickel element is introduced after forming theunderlying layer to hold the nickel element in contact on the surface ofthe underlying layer.

In the present embodiment, acetate nickel aqueous solution containing 10ppm nickel (in terms of weight) is applied to the surface of theunderlying film to introduce the nickel element and an amorphous siliconfilm is formed on the surface. Then, a crystal silicon film 173 in whichthe concentration of nickel has been reduced is obtained as shown inFIG. 30E by implementing the other processes in the same manner with thecase of the 28-th embodiment. Beside the method of using the solution,sputtering, CVD or adsorption may be used as the method for introducingnickel element, i.e. a metal element which promotes crystallization ofsilicon.

[36-th Embodiment]

A 36-th embodiment relates to a case of improving the crystallinity ofan island pattern formed of a crystal silicon film obtained byirradiating laser light in the state shown in FIG. 31E, the state shownin FIG. 32A or the state shown in FIG. 33A. According to the presentembodiment, a predetermined annealing effect can be obtained withrelatively low irradiation energy density by irradiating the laser lightin the state shown in FIGS. 31E, 32A and 33A. This effect is consideredto have been effected because the laser energy is irradiated to a spotof small area, thus enhancing the efficiency of energy utilized in theannealing.

[37-th Embodiment]

A 37-th embodiment relates to a case in which patterning of an activelayer of a thin film transistor is devised in order to enhance theeffect of annealing by the irradiation of laser light. FIGS. 35A through35F show a process for fabricating the thin film transistor according tothe present embodiment.

At first, a silicon oxide film or silicon oxynitride film 251 is formedas an underlying layer on a Corning 1737 glass substrate 250. Next, anamorphous silicon film is formed in a thickness of 500 angstrom by usingthe low pressure thermal CVD. It is noted that this amorphous siliconfilm turns out to be a crystal silicon film 252 through thecrystallization process described below.

Next, the amorphous silicon film is crystallized by the method shown inthe 28-th and 29-th embodiments (FIGS. 30 and 31) to obtain the crystalsilicon film. Thus, the state shown in FIG. 35A is obtained. After that,the crystal silicon film 252 is formed on the glass substrate inaccordance to the process shown in the 28-th and 29-th embodiments. Thatis, the amorphous silicon film is crystallized by the heat treatmentusing nickel element to obtain the crystal silicon film 252. The heattreatment is implemented at 620° C. for four hours. The processdescribed below applies to the both crystal silicon films fabricated inaccordance to the processes of the 28-th and 29-th embodiments.

After obtaining the crystal silicon film, a pattern 253 for constructingan active layer of a thin film transistor is formed. At this time, thepattern is formed so as to have a profile 254 shown in FIG. 35B in orderto suppress the shape of the pattern from being deformed in the latertreatment step of irradiating laser light.

In general, when laser light is irradiated to a pattern 258 made from anormal island-shape silicon film formed on a base 257 as shown in FIG.36A, a convex portion 260 is formed at the edge of a pattern 259 afterthe irradiation of the laser light as shown in FIG. 36B. It isconsidered to happen because energy of the irradiated laser light isconcentrated at the edge of the pattern where heat cannot be released.

The above-mentioned phenomenon may become a factor of defective wirescomposing a thin film transistor or of defective operation thereoflater. Then, the pattern 235 of the active layer is formed so as to havethe profile as shown in FIG. 35B in the formation of the presentembodiment. Such formation allows to suppress the pattern of the siliconfilm from being deformed like the one shown in FIG. 36B when laser lightis irradiated.

It is preferable to set an angle of the part designated by the referencenumeral 254 from 20° to 50°. It is not preferable to set the angle 254below 20° because an area occupied by the active layer increases and itbecomes difficult to form it. Further, it is not also preferable to setthe angle 254 above 50° because the effect for suppressing the shape asshown in FIG. 36B from being formed drops.

The pattern 253 may be realized by utilizing isotropic dry etching andby controlling the conditions of this dry etching in patterning it.After obtaining the pattern (which turns out to be the active layerlater) having the shape 253 in FIG. 35B, laser light is irradiated asshown in FIG. 35C. This step allows to diffuse the nickel element whichis locally gathered within the pattern 253 and to promote thecrystallization of the pattern.

After finishing to irradiate laser light, a heat treatment isimplemented within an oxygen atmosphere containing 3 volume % of HCl toform a thermal oxide film 255. The thermal oxide film is formed in 100angstrom thick by implementing the heat treatment for 12 hours in theoxygen atmosphere containing HCl at 650° C. The nickel element containedin the pattern 253 is gettered to the thermal oxide film by the actionof chlorine. At this time, because the block of the nickel element hasbeen destroyed and diffused through the irradiation of laser light inthe previous step, the gettering of the nickel element is effectivelyperformed.

Further, the gettering is performed also from the side of the pattern253 when the formation as shown in the present embodiment is adopted.This is useful in enhancing the OFF current characteristics and thereliability of the thin film transistor finally completed. It is becausethe existence of metal element which is typified by nickel element whichpromotes crystallization of silicon and which exists in the side of theactive layer exerts a wide influence over the increase of OFF currentand the instability of the characteristics.

After forming the thermal oxide film 255 for gettering as shown in FIG.35D, it is eliminated. Thus, the state shown in FIG. 35E is obtained. Itis concerned that the silicon oxide film 251 might be etched in the stepof eliminating the thermal oxide film 255 when the silicon oxide film isadopted as the underlying layer. However, it does not matter so muchwhen the thickness of the thermal oxide film 255 is as thin as 100angstrom as shown in the present embodiment.

After obtaining the state shown in FIG. 35E, a new thermal oxide film256 is formed in a thickness of 100 angstrom by a heat treatment in anatmosphere of 100% oxygen at 650° C. The thermal oxide film 256 iseffective in suppressing the surface of the pattern 253 from beingroughened when the laser light is irradiated later. The thermal oxidefilm 256 also forms a part of a gate insulating film later.

Because the thermal oxide film 256 has a very excellent interfacialcharacteristic with the crystal silicon film, it is useful to utilize itas part of the gate insulating film. The laser light may be irradiatedagain after forming the thermal oxide film 256. Thus, the crystalsilicon film 253 from which the concentration of nickel element has beenreduced and which has a high crystallinity may be obtained. Thereafter,the thin film transistor is fabricated by going through the processshown in FIG. 32 or 33.

[38-th Embodiment]

A 38-th embodiment relates to a case devised in applying a heattreatment at a temperature more than a distortion point of a glasssubstrate. It is preferable to perform the process for gettering themetal element which promotes crystallization of silicon in the presentinvention at a high temperature as much as possible.

When the Corning 1737 glass substrate (distortion point: 667° C.) isused for instance, the higher gettering effect can be obtained when thetemperature in gettering nickel element by forming the thermal oxidefilm is 700° C. rather than when it is 650° C. However, if the heatingtemperature for forming the thermal oxide film is set at 700° C. whileusing the Corning 1737 glass substrate, the glass substrate deforms as aresult.

The present embodiment is to solve this problem. That is, according tothe arrangement shown in the present embodiment, the glass substrate isplaced on a lapping plate which is formed of quartz whose flatness isguaranteed and the heat treatment is implemented in this state. Thereby,the flatness of the softened glass substrate is maintained by theflatness of the lapping plate. It is noted that it is also important toimplement cooling also in the state in which the glass substrate isplaced on the lapping plate. The adoption of such arrangement allows theheat treatment to be implemented even at a temperature more than thedistortion point of the glass substrate.

[39-th Embodiment]

A 39-th embodiment relates to a case of obtaining a crystal silicon filmon a glass substrate by utilizing nickel element. In the presentembodiment, the crystal silicon film having a high crystallinity isobtained by the action of nickel element at first. Then, laser light isirradiated to enhance the crystallinity of the film and to diffuse thenickel element which is concentrated locally within the film, i.e. toextinguish blocks of nickel.

Next, an oxide film is formed on the crystal silicon film by thermaloxidation. At this time, the nickel element remaining in the crystalsilicon film is gettered to the thermal oxide film. At this time,because the nickel element is dispersed therein by the irradiation oflaser light, the gettering proceeds effectively. Then, the thermal oxidefilm containing the nickel element in high concentration as a result ofthe gettering is eliminated. Thereby, the crystal silicon film in whichthe concentration of nickel element is low, while having the highcrystallinity, is obtained on the glass substrate.

FIGS. 37A through 37E are diagrams showing the fabrication processaccording to present embodiment. At first, a silicon oxynitride film 262is formed as an underlying film in a thickness of 3000 angstrom on theglass substrate 261 of Corning 1737 (distortion point: 667° C.). Thesilicon oxynitride film is formed by using the plasma CVD using silane,N₂O gas and oxygen as original gases in the present embodiment. Theplasma CVD using TEOS gas and N₂O gas may be also used instead of that.

The silicon oxynitride film has a function of suppressing the diffusionof impurities from the glass substrate in the later steps (seeing fromthe level of fabrication of a semiconductor, the glass substratecontains a large amount of impurities). It is noted that although thesilicon nitride film is the most suitable to obtain the function forsuppressing the diffusion of the impurities in maximum, the siliconnitride film is not practical because it is peeled off from the glasssubstrate due to the influence of stress. A silicon oxide film may bealso used as the underlying film.

It is also important to increase the hardness of the underlying film 262as much as possible. It is concluded from the fact that the harder thehardness of the underlying film (i.e. the smaller the etching ratethereof), the higher the reliability is in an endurance test of the thinfilm transistor obtained in the end The reason thereof is assumed to becaused by the effect of blocking the impurities from the glass substratein the fabrication process of the thin film transistor.

It is also effective to contain a small amount of halogen elementtypified by chlorine in the underlying film 262. Thereby, the metalelement which promotes crystallization of silicon and exists within thesemiconductor layer may be gettered by the halogen element in the laterstep. It is also effective to add a hydrogen plasma treatment afterforming the underlying film. It is also effective to implement a plasmatreatment in an atmosphere in which oxygen and hydrogen are mixed. Thesetreatments are effective in eliminating carbon component which isadsorbed on the surface of the underlying film and in enhancing thecharacteristic of interface with a semiconductor film formed later.

Next, an amorphous silicon film 263, which turns out to be a crystalsilicon film later, is formed in a thickness of 500 angstrom by the lowpressure thermal CVD. The reason why the low pressure thermal CVD isused is because thereby, the quality of the crystal silicon filmobtained later is better, i.e. the film quality is denser in concrete.Beside the low pressure thermal CVD, the plasma CVD or the like may beused.

The amorphous silicon film fabricated here is desirable to have 5×10¹⁷cm⁻³ to 2×10¹⁹ cm⁻³ of concentration of oxygen within the film. It isbecause oxygen plays an important role in the later step of getteringthe metal element which promotes crystallization of silicon. However, itmust be careful here because the crystallization of the amorphoussilicon film is hampered if the oxygen concentration is higher than theabove-mentioned range of concentration. The concentration of otherimpurities such as those of nitrogen and carbon is preferred to be lowto the utmost. In concrete, the concentration must be below 2×10¹⁹ cm⁻³.

The upper limit of the thickness of the amorphous silicon film is about2000 angstrom. It is because a thick film is disadvantageous inobtaining the effect of the irradiation of laser light implementedlater. That is, the most of laser light irradiated to a silicon film isabsorbed by the surface of the film. The lower limit of the thickness ofthe amorphous silicon film 263 is practically about 200 angstrom, thoughit depends on a method for forming the film.

Next, nickel element is introduced to the amorphous silicon film 263 tocrystallize it. Here, the nickel element is introduced by applyingnickel acetate solution containing 10 ppm (weight conversion) of nickelon the surface of the amorphous silicon film 263. Beside the method ofusing the above-mentioned solution, sputtering, CVD, plasma treatment oradsorption may be used as the method for introducing the nickel element.Among them, the method of using the solution is useful in that it issimple and that the concentration of the metal element may be readilyadjusted.

The nickel acetate solution is applied as described above to form awater film (liquid film) 264 of the nickel acetate aqueous solution asshown in FIG. 37A. After obtaining this state, extra solution is blownout by using a spin coater not shown. Thus, the nickel element is heldin contact on the surface of the amorphous silicon film 263. It is notedthat it is preferable to use nickel sulfate solution, instead of thenickel acetate, if the remained impurities in the later heating processis taken into consideration. It is because the nickel acetate aqueoussolution contains carbon and it might be carbonized in the later heatingprocess, thus remaining within the film. An amount of the nickel elementto be introduced may be controlled by adjusting the concentration ofnickel element within the solution.

Next, a heat treatment is implemented in the temperature range from 550°C. to 650° C. in the state shown in FIG. 37B to crystallize theamorphous silicon film 263 and to obtain a crystal silicon film 265.This heat treatment is implemented in a reducing atmosphere. Thetemperature of this heat treatment is preferable to be a temperaturebelow the distortion point of the glass substrate. Because thedistortion point of the Corning 1737 glass substrate is 667° C., it ispreferable to set the upper limit of the heating temperature here atabout 650° C., leaving some margin.

In the present embodiment, the heat treatment is implemented for fourhours at 620° C. within a nitrogen atmosphere containing 3 volume % ofhydrogen. The reason why the reducing atmosphere is adopted in thecrystallization step in a way of the heat treatment is to prevent oxidesfrom being created in the step of the heat treatment and moreconcretely, to suppress nickel from reacting with oxygen and NiOx frombeing created on the surface of the film or within the film.

Oxygen couples with nickel and contributes a lot in gettering nickel inthe later gettering step. However, it has been found that if oxygencouples with nickel in the above-mentioned stage of the crystallization,it hampers the crystallization. Accordingly, it is important to suppressthe oxides from being created to the utmost in the crystallization stepin a way of heating.

The concentration of oxygen within the atmosphere for implementing theheat treatment for the crystallization has to be in an order of ppm, orpreferably, less than 1 ppm. Inert gases such as argon, beside nitrogen,or their mixed gas may be used as the gas which occupies the most of theatmosphere for implementing the heat treatment for the crystallization.

After the crystallization step by the heat treatment described above,nickel element remains in certain blocks. It has been confirmed by anobservation through a TEM (Transmission type Electron Microscope). Whilethe reason why nickel exists in certain blocks has not been clarifiedyet, it is considered to be related with some crystallization mechanism.

Next, laser light is irradiated as shown in FIG. 37C. Here, KrF excimerlaser (wavelength: 248 nm) is used. The irradiation is implemented byscanning the laser beam whose shape is linear. The irradiation of thelaser light allows the nickel element which has been concentratedlocally as a result of the crystallization by means of theabove-mentioned heat treatment to be distributed within the film 265 insome degree. That is, it allows to extinguish the blocks of nickelelement and to distribute the nickel element.

Another heat treatment is implemented in a step in FIG. 37D to form athermal oxide film for gettering the nickel element. This heat treatmentis implemented within an atmosphere of 100% oxygen at 640° C. for 12hours. As a result of this step, the thermal oxide film is formed in athickness of 100 angstrom.

This step is carried out to eliminate the nickel element (or other metalelement which promotes crystallization of silicon) which has beenintroduced intentionally for the crystallization in the initial stagefrom the crystal silicon film 265. This heat treatment is implemented ata temperature higher than that of the heat treatment implemented for thecrystallization described above. It is an important condition foreffectively implementing the gettering of nickel element. It is notedthat although this heat treatment may be implemented in the same or lesstemperature in the heat treatment implemented for the crystallization,it is less effective.

This heat treatment is implemented in the temperature range from 600° C.to 750° C. upon meeting the above-mentioned condition. The effect forgettering nickel element may be obtained remarkably in this step if thetemperature is higher than 600° C. In this step, the nickel elementwhich has been distributed by the irradiation of laser light describedabove is gettered effectively to the oxide film. The upper limit of theheating temperature is limited by the distortion point of the glasssubstrate to be used.

It must be careful not to implement the heat treatment in a temperatureabove the distortion point of the glass substrate to be used because,otherwise, it is deformed. It is noted, in this regard, that the heattreatment may be implemented above the distortion point of the glasssubstrate to be used by placing the glass substrate on the lapping platemade from quartz for example whose flatness is guaranteed and byimplementing the heat treatment in this state as described in the 38-thembodiment.

Because the thermal oxide film 266 is formed, the thickness of thecrystal silicon film 263 reaches to about 450 angstrom. When the heatingtemperature is 600° C. to 750° C. in the heat treatment, the treatmenttime (heating time) is set at 10 hours to 48 hours or typically at 24hours. This treatment time may be set adequately depending on thethickness of the oxide film to be obtained as a matter of course. Inthis gettering, oxygen existing within the crystal silicon film plays animportant role. That is, the gettering of nickel element proceeds in amanner of nickel oxide formed when oxygen couples with nickel.

If the concentration of oxygen is too much, it becomes the factor ofhampering the crystallization of the amorphous silicon film 263 in thecrystallization step shown in FIG. 37B as described above. However, theexistence thereof plays an important role in the process of getteringnickel as described above. Accordingly, it is important to control theconcentration of oxygen existing within the amorphous silicon film, thestarting film. Further, because the nickel element is gettered to theoxide film to be formed in the aforementioned step, naturally the nickelconcentration within the oxide film becomes high as compared to otherregions.

Further, it has been observed that the concentration of nickel elementis apt to be high near the interface between the crystal silicon film265 and the thermal oxide film 266. It is considered to happen becausethe region where the gettering mainly takes place is on the side of theoxide film near the interface between the crystal silicon film and theoxide film. The gettering proceeding near the interface is considered tobe caused by the existence of stress and defects near the interface.

Next, the thermal oxide film 266 containing nickel in high concentrationis eliminated. While the thermal oxide film 266 may be eliminated bymeans of dry etching or wet etching using buffer hydrofluoric acid orother hydrofluorite (hydrofluoric) etchant, it is implemented by the wetetching by using the buffer hydrofluoric acid in the present embodiment.Thus, a crystal silicon film 267 in which the concentration of nickelhas been reduced is obtained as shown in FIG. 37E.

Because nickel element is contained near the surface of the obtainedcrystal silicon film 267 relatively in high concentration, it iseffective to advance the etching of the above-mentioned oxide film 266to over-etch, more or less, the surface of the crystal silicon film 267.It is also effective to irradiate laser light again after eliminatingthe thermal oxide film 266 to promote the crystallization of theobtained crystal silicon film 267 further. That is, it is effective toirradiate the laser light again after gettering nickel element.

KrF excimer laser (wavelength: 248 nm) is used as the laser light to beused in the present embodiment. However, XeCl excimer laser (wavelength:308 nm) and other types of lasers may be also used. Further, it ispossible to arrange so as to irradiate ultraviolet ray or infrared ray,instead of the laser light.

[40-th Embodiment]

A 40-th embodiment relates to a case when Cu is used as the metalelement which promotes crystallization of silicon in the arrangementshown in the 39-th embodiment. In this case, cupric acetate[Cu(CH₃COO)₂] and cupric chloride (CuCl₂ 2H₂O) may be used as thesolution for introducing Cu. The latter is used in the presentembodiment. A crystal silicon film 267 in which the concentration ofcopper content has been reduced is obtained as shown in FIG. 37E byimplementing other processes in the same manner with the 39-thembodiment.

[41-st Embodiment]

A 41-st embodiment relates to a case of growing crystal in the formdifferent from that in the 39-th embodiment. That is, the presentembodiment relates to a method of growing the crystal in a directionparallel to the substrate, i.e. a method called lateral growth, byutilizing the metal element which promotes crystallization of silicon.

FIGS. 38A through 38E show the fabrication process according to the41-st embodiment. At first, a silicon oxynitride film is formed as anunderlying film 269 in a thickness of 3000 angstrom on the Corning 1737glass substrate 268. It is noted that a quartz substrate may be usedinstead of the glass substrate. Next, an amorphous silicon film 270which is the starting film of a crystal silicon film is formed in athickness of 600 angstrom by low pressure thermal CVD. The thickness ofthe amorphous silicon film is preferable to be less than 2000 angstromas described before. It is noted that plasma CVD may be also usedinstead of the low pressure thermal CVD.

Next, a silicon oxide film not shown is formed in a thickness of 1500angstrom and is patterned to form a mask 271. An opening is created onthe mask in a region 272. The amorphous silicon film 270 is exposed atthe region where the opening 272 is created. The opening 272 has a thinand long rectangular shape in the longitudinal direction from the depthto the front side of the figure. Preferably, the width of the opening272 is 20 μm or more. The length thereof in the longitudinal directionmay be determined as necessary.

Then, the nickel acetate aqueous solution containing 10 ppm of nickelelement in terms of weight is applied in the same manner with the 40-thembodiment and the extra solution is removed by implementing spin dryingby using a spinner not shown. Thus, the solution is held in contact onthe exposed surface of the amorphous silicon film 270 as indicated by adot line 273 in FIG. 38A.

Next, a heat treatment is implemented at 640° C. for four hours in anitrogen atmosphere containing 3 volume % of hydrogen and in whichoxygen is minimized. Then, crystal grows in the direction parallel tothe substrate 268 as indicated by the reference numeral 274 in FIG. 38B.This crystal growth advances from the region of the opening 272 to whichnickel element has been introduced to the surrounding part. This crystalgrowth in the direction parallel to the substrate will be referred to aslateral growth throughout the present specification.

It is possible to advance this lateral growth across more than 100 μmunder the conditions shown in the present embodiment. Then, a siliconfilm 275 having the region in which the crystal has thus grown laterallyis obtained. It is noted that crystal growth in the vertical directioncalled vertical growth advances from the surface of the silicon film tothe underlying interface in the region where the opening 272 is formed.

Then, the mask 271 made from the silicon oxide film for selectivelyintroducing nickel element is eliminated. Thus, the state shown in FIG.38C is obtained. In this state, the vertically grown region, thelaterally grown region and a region in which no crystal has grown(having amorphous state) exist within the silicon film 275. In thisstate, nickel element is unevenly distributed within the film.Specifically, the nickel element exists relatively in high concentrationin the region where the opening 272 is created and in the edge portionof the crystal growth indicated by the reference numeral 274.

After obtaining the state shown in FIG. 38C, laser light is irradiated.Here, KrF excimer laser is irradiated in the same manner with the 39-thembodiment to diffuse the nickel element unevenly distributed to createa condition in which gettering may be implemented readily in the latergettering step. After irradiating the laser light, a heat treatment isimplemented at 650° C. for 12 hours within an atmosphere of 100% oxygen.

In this step, an oxide film 276 containing nickel element in highconcentration is formed. In the same time, the concentration of nickelelement within the silicon film 275 may be reduced relatively. Here, thethermal oxide film 276 is formed in a thickness of 100 angstrom. Thenickel element gettered when the thermal oxide film has been formed iscontained in the thermal oxide film in high concentration.

Further, because the thermal oxide film 276 is formed, the thickness ofthe crystal silicon film 275 is reduced to about 500 angstrom. Next, thethermal oxide film 276 containing nickel element in high concentrationis eliminated. In the crystal silicon film of this state, the nickelelement has a distribution of concentration such that it exists in highconcentration toward the surface of the crystal silicon film. This iscaused by the fact that the nickel element has been gettered to thethermal oxide film when the thermal oxide film 276 has been formed.

Accordingly, it is useful to etch the surface of the crystal siliconfilm to eliminate the region in which the nickel element exists in highconcentration after eliminating the thermal oxide film 276. That is, thecrystal silicon film in which the nickel element concentration isreduced further may be obtained by etching the surface of the crystalsilicon film in which the nickel element exists in high concentration.However, in this case, the thickness of the silicon film finallyobtained needs to be taken into consideration.

Next, patterning is implemented to form a pattern 277 formed of thelaterally grown region. The concentration of nickel element whichremains within the pattern 277 made from the laterally grown region thusobtained may be reduced further as compared to the case shown in the39-th embodiment. This is caused by the fact that the concentration ofthe metal element contained within the laterally grown region is loworiginally. In concrete, the concentration of nickel element within thepattern 277 made from the laterally grown region may be readily reducedto the order of 10¹⁷ cm⁻³ or less.

When a thin film transistor is formed by utilizing the laterally grownregion, a semiconductor device having a higher mobility may be obtainedas compared to the case when the vertically grown region as shown in the39-th embodiment (crystal grows vertically on the whole surface in thecase of the 39-th embodiment) is utilized. It is noted that it is usefulto implement the etching process further after forming the pattern shownin FIG. 38E to eliminate the nickel element existing on the surface ofthe pattern.

Next, a thermal oxide film 278 is formed after forming the pattern 277as described above. This thermal oxide film 278 is formed in a thicknessof 100 angstrom by implementing a heat treatment for 12 hours in anoxygen atmosphere at 650° C. This thermal oxide film becomes a part of agate insulating film later in constructing a thin film transistor. Whenthe thin film transistor is fabricated after that, a silicon oxide filmis formed by plasma CVD or the like over the thermal oxide film 278 toform the gate insulating film together with the thermal oxide film 278.

[42-nd Embodiment]

A 42-nd embodiment relates to a case of fabricating a thin filmtransistor disposed in a pixel region of an active matrix type liquidcrystal display or an active matrix type EL display. FIGS. 39A through39E show the fabrication process according to the present embodiment.

At first, the crystal silicon film is formed on the glass substratethrough the process shown in the 39-th or 41-st embodiment. When thecrystal silicon film is obtained through the arrangement shown in the39-th embodiment, it is patterned and the state shown in FIG. 39A isobtained. In FIG. 39A, the reference numeral (280) denotes a glasssubstrate, (281) an underlying film, and (282) an active layer formed ofthe crystal silicon film. After obtaining the state shown in FIG. 39A, aplasma treatment is implemented within a reduced pressure atmosphere inwhich oxygen and hydrogen are mixed. The plasma is generated byhigh-frequency discharge.

Organic substances existing on the surface of the exposed active layer282 may be removed by the plasma treatment. Specifically, the organicsubstances adsorbing on the surface of the active layer are oxidized byoxygen plasma and the oxidized organic substances are reduced andvaporized by hydrogen plasma. Thus the organic substances existing onthe surface of the exposed active layer 282 are removed. The removal ofthe organic substances is very effective in suppressing fixed chargefrom existing on the surface of the active layer 282. That is, becausethe fixed charge caused by the existence of organic substances hampersthe operation of the device and renders the characteristics thereofinstable, it is very useful to remove it.

After removing the organic substances, thermal oxidation is implementedwithin an oxygen atmosphere at 640° C. to form a thermal oxide film 279of 100 angstrom thick. This thermal oxide film has a high interfacialcharacteristic with a semiconductor layer and composes a part of a gateinsulating film later. Thus, the state shown in FIG. 39A is obtained.After that, a silicon oxynitride film 283 which composes the gateinsulating film is formed in a thickness of 1000 angstrom. While thefilm may be formed by using the plasma CVD using mixed gas of silane andN₂O and oxygen or the plasma CVD using mixed gas of TEOS and N₂O, theformer is used in the present embodiment.

The silicon oxynitride film 283 functions as the gate insulating filmtogether with the thermal oxide film 279. It is also effective tocontain halogen element within the silicon oxynitride film. That is, itis possible to prevent the function of the gate insulating film as aninsulating film from dropping by the influence of the nickel element (oranother metal element which promotes crystallization of silicon)existing within the active layer by fixing the nickel element by theaction of the halogen element.

It is significant to use the silicon oxynitride film as the insulatingfilm in that metal element hardly infiltrates to the gate insulatingfilm from its dense film quality. If metal element infiltrates into thegate insulating film, its function as an insulating film drops, thuscausing instability and dispersion of characteristics of the thin filmtransistor. It is noted that a silicon oxide film which is normally usedmay be also used for the gate insulating film.

After forming the silicon oxynitride film 283 which functions as thegate insulating film, an aluminum film (not shown. It turns out to be apattern 284 after patterning described later) which functions as a gateelectrode later is formed by sputtering. 0.2 weight % of scandium isincluded within the aluminum film to suppress hillock and whisker frombeing produced in the later process. The hillock and whisker mean thatabnormal growth of aluminum occurs by heating, thus forming needle orprickle-like projections.

After forming the aluminum film, a dense anodic oxide film not shown isformed. The anodic oxide film is formed by using ethylene glycolsolution containing 3 weight % of tartaric acid as electrolyte. That is,the anodic oxide film having the dense film quality is formed on thesurface of the aluminum film by setting the aluminum film as the anodeand platinum as the cathode and by anodizing within this electrolyte.The thickness of the anodic oxide film not shown having the dense filmquality is around 100 angstrom. This anodic oxide film plays a role ofenhancing the adhesiveness with a resist mask to be formed later. It isnoted that the thickness of the anodic oxide film may be controlled byadjusting voltage applied during the anodization.

Next, the resist mask 285 is formed and the aluminum film is patternedso as to have a pattern 284. The state shown in FIG. 39B is thusobtained. Here, another anodization is implemented. In this case, 3weight % of oxalate aqueous solution is used as electrolyte. A porousanodic oxide film 287 is formed by anodizing within this electrolyte bysetting the aluminum pattern 284 as the anode.

In this step, the anodic oxide film 287 is formed selectively on thesides of the aluminum pattern 284 because the resist mask 285 having thehigh adhesiveness exists thereabove. The anodic oxide film may be grownup to several μm thick. The thickness is 6000 angstrom in the presentembodiment. It is noted that the range of growth may be controlled byadjusting an anodizing time.

Next, the resist mask 285 is removed. Then, a dense anodic oxide film isformed again. That is, the anodization is implemented again by using theethylene glycol solution containing 3 weight % of tartaric acid aselectrolyte. Then, an anodic oxide film 288 having a dense film qualityis formed because the electrolyte infiltrates into the porous anodicoxide film 287. This dense anodic oxide film 288 is 1000 angstrom thick.The thickness is controlled by adjusting applied voltage.

Here, the exposed silicon oxynitride film 283 and the thermal oxide film279 are etched by utilizing dry etching. Then, the porous anodic oxidefilm 287 is eliminated by using mixed acid in which acetic acid, nitricacid and phosphoric acid are mixed. Thus, the state shown in FIG. 39D isobtained. After obtaining the state shown in FIG. 39D, impurity ions areinjected. Here, P (phosphorus) ions are injected by plasma doping inorder to fabricate an N-channel type thin film transistor. In this step,heavily doped regions 290 and 294 and lightly doped regions 291 and 293are formed because part of the remaining silicon oxide film 289functions as a semi-permeable mask, thus blocking part of the injectedions.

Next, laser light or intense light is irradiated to activate the regionsinto which the impurity ions have been injected. The irradiation isimplemented by using an infrared lamp here. Thus, a source region 290, achannel forming region 292, a drain region 294 and low concentrationimpurity regions 291 and 293 are formed in a manner of self-alignment.The one designated by the reference numeral 293 here is the regioncalled the LDD (lightly doped drain).

It is noted that when the dense anodic oxide film 288 is formed so thickto be more than 2000 angstrom, an offset gate region may be formed onthe outside of the channel forming region 292 by its thickness. Althoughthe offset gate region is formed also in the present embodiment, it isnot shown in the figures because its size is small, its contribution dueto the existence thereof is small and the figures might otherwise becomecomplicated.

Next, a silicon oxide film or a silicon nitride film or their laminatedfilm is formed as an interlayer insulating film 295. The silicon nitridefilm is used in the present embodiment. The interlayer insulating film295 may be constructed by forming a layer made from a resin material onthe silicon oxide film or the silicon nitride film. Then, contact holesare created to form a source electrode 296 and a drain electrode 297.Thus, the thin film transistor shown in FIG. 39E is completed.

[43-rd Embodiment]

A 43-rd embodiment is related to a method for forming the gateinsulating film 283 in the arrangement shown in the 42-nd embodiment.Thermal oxidation may be used as a method for forming the gateinsulating film when a quartz substrate or a glass substrate having ahigh heat resistance is used as the substrate. In the presentembodiment, the thermal oxidation is used to form the gate insulatingfilm 283 and the thin film transistor having the structure shown in FIG.39E is obtained by implementing the other process in the same mannerwith the 42-nd embodiment.

The thermal oxidation allows the film quality to be densified and hence,is useful in obtaining a thin film transistor having stablecharacteristics. That is, because an oxide film formed by the thermaloxidation is dense as an insulating film and movable electric chargeexisting therein can be reduced, it is one of the most suitable films asa gate insulating film.

[44-th Embodiment]

A 44-th embodiment relates to a case of fabricating a thin filmtransistor through a process different from that shown in FIG. 39. FIGS.40A through 40E show the fabrication process according to the presentembodiment. At first, the crystal silicon film is formed on the glasssubstrate through the process shown in the 39-th embodiment (FIG. 37) orin the 41-st embodiment (FIG. 38). It is then patterned, thus obtainingthe state shown in FIG. 40A.

After obtaining the state shown in FIG. 40A, a plasma treatment isimplemented within the reduced pressure atmosphere in which oxygen andhydrogen are mixed. In the state shown in FIG. 40A, the referencenumeral (299) denotes a glass substrate, (300) an underlying film, (301)an active layer formed of the crystal silicon film and (298) a thermaloxide film formed again after eliminating the thermal oxide film forgettering. After obtaining the state shown in FIG. 40A, a siliconoxynitride film 302 which composes a gate insulating film is formed in athickness of 1000 angstrom. While the film may be formed by using theplasma CVD using mixed gas of oxygen, silane and N₂O or the plasma CVDusing mixed gas of TEOS and N₂O, the former is used here.

The silicon oxynitride film 302 composes the gate insulating filmtogether with the thermal oxide film 298. It is noted that a siliconoxide film may be used beside the silicon oxynitride film. After formingthe silicon oxynitride film 302 which functions as the gate insulatingfilm, an aluminum film (not shown. It turns out to be a pattern 304after patterning described later) which functions as a gate electrodelater is formed by sputtering. 0.2 weight % of scandium is includedwithin the aluminum film.

After forming the aluminum film, a dense anodic oxide film not shown isformed. The anodic oxide film is formed by using ethylene glycolsolution containing 3 weight % of tartaric acid as electrolyte. That is,the anodic oxide film having the dense film quality is formed on thesurface of the aluminum film by setting the aluminum film as the anodeand platinum as the cathode and by anodizing within this electrolyte.The thickness of the anodic oxide film not shown having the dense filmquality is around 100 angstrom. This anodic oxide film plays a role ofenhancing the adhesiveness with a resist mask to be formed later. It isnoted that the thickness of the anodic oxide film may be controlled byadjusting voltage applied during the anodization.

Next, the resist mask 303 is formed and the aluminum film is patternedso as to have the pattern 304. Here, another anodization is implemented.In this case, 3 weight % of oxalate aqueous solution is used aselectrolyte. A porous anodic oxide film 305 is formed by anodizingwithin this electrolyte by setting the aluminum pattern 304 as theanode.

In this step, the anodic oxide film 305 is formed selectively on thesides of the aluminum pattern because the resist mask 303 having thehigh adhesiveness exists thereabove. The anodic oxide film 305 may begrown up to several μm thick. The thickness is 6000 angstrom in thepresent embodiment. It is noted that the range of growth may becontrolled by adjusting an anodizing time.

Next, after removing the resist mask 303, a dense anodic oxide film isformed again. That is, the anodization is implemented again by using theethylene glycol solution containing 3 weight % of tartaric acid aselectrolyte. Then, an anodic oxide film 306 having a dense film qualityis formed because the electrolyte infiltrates into the porous anodicoxide film 305. Here, the initial injection of impurity ions isimplemented. This step may be implemented after removing the resist mask303. A source region 307 and a drain region 309 are formed by injectingthe impurity ions. It is noted that no impurity ion is injected to aregion 308 at this time.

Next, the porous anodic oxide film 305 is eliminated by using mixed acidin which acetic acid, nitric acid and phosphoric acid are mixed. Thus,the state shown in FIG. 40D is obtained. After obtaining the state shownin FIG. 40D, impurity ions are injected again. The impurity ions areinjected under the doping condition lighter than that of the firstinjection. In this step, lightly doped regions 310 and 311 are formedand a region 312 turns out to be a channel forming region.

Then, laser light or intense light is irradiated to activate the regionsinto which the impurity ions have been injected. The laser light is usedhere. Thus, the source region 307, the channel forming region 312, thedrain region 309 and low concentration impurity regions 310 and 311 areformed in a manner of self-alignment. Here, the one designated by thereference numeral 311 is the region called the LDD (lightly dopeddrain).

Next, while a silicon oxide film or a silicon nitride film or theirlaminated film is formed as an interlayer insulating film 313, thelaminated film of the silicon oxide film and the silicon nitride film isformed here. The interlayer insulating film may be also constructed byforming a layer made from a resin material on the silicon oxide film orthe silicon nitride film. After that, contact holes are created to forma source electrode 314 and a drain electrode 315. Thus, the thin filmtransistor shown in FIG. 40E is completed.

[45-th Embodiment]

A 45-th embodiment relates to a case when an N-channel type thin filmtransistor and a P-channel type thin film transistor are formed in acomplementary manner. FIGS. 41A through 41F are diagrams showing afabrication process according to the present embodiment. The formationshown in the present embodiment may be utilized for various thin filmintegrated circuits integrated on an insulating surface as well as forperipheral driving circuits of an active matrix type liquid crystaldisplay for example.

At first, a silicon oxide film or a silicon oxynitride film is formed asan underlying film 318 on a glass substrate 317 as shown in FIG. 41A. Itis preferable to use the silicon oxynitride film, and it is used in thepresent embodiment. Next, while an amorphous silicon film not shown isformed by the plasma CVD or the low pressure thermal CVD, the latter isused here. Then, the amorphous silicon film is transformed into acrystal silicon film by the method shown in the 39-th embodiment.

Next, a plasma treatment is implemented within an atmosphere in whichoxygen and hydrogen are mixed and the crystal silicon film thus obtainedis patterned to obtain active layers 319 and 320. Thus, the state shownin FIG. 41A is obtained. Further, a heat treatment is implemented at650° C. for 10 hours within a nitrogen atmosphere containing 3 volume %of HCl in the state shown in FIG. 41A in order to suppress the influenceof carriers moving the sides of the active layers.

Because an OFF current characteristic becomes bad if a trap level existsdue to the existence of metal element on the sides of the active layers,it is useful to implement the above-mentioned treatment to lower thedensity of the level on the sides of the active layers. Next, a thermaloxide film 316 and a silicon oxynitride film 321 which compose a gateinsulating film are formed. When quartz is used as the substrate here,it is desirable to compose the gate insulating film only by the thermaloxide film formed by using the above-mentioned thermal oxidation.

Next, an aluminum film (not shown. It turns out to be a pattern afterpatterning thereof described later) which composes a gate electrodelater is formed in a thickness of 4000 angstrom. Beside the aluminumfilm, a metal which can be anodized, such as tantalum, may be used.After forming the aluminum film, a very thin and dense anodic oxide filmis formed on the surface thereof by the method described before.

Next, a resist mask not shown is placed on the aluminum film to patternthe aluminum film. Then, anodization is implemented by setting theobtained aluminum pattern as the anode to form porous anodic oxide films324 and 325. The thickness of the porous anodic oxide films is 5000angstrom. Then, another anodization is implemented under the conditionof forming dense anodic oxide films 326 and 327. The thickness of thedense anodic oxide films 326 and 327 is 800 angstrom. Thus, the stateshown in FIG. 41B is obtained. Next, the exposed silicon oxynitride film321 and the thermal oxide film 316 are eliminated by dry etching, thusobtaining the state shown in FIG. 41C as a result.

After obtaining the state shown in FIG. 41C, the porous anodic oxidefilms 324 and 325 are eliminated by using mixed acid in which aceticacid, nitric acid and phosphoric acid are mixed. Thus, the state shownin FIG. 41D is obtained. Here, resist masks are disposed alternately toinject P (phosphorus) ions to the thin film transistor on the left sideand B (boron) ions to the thin film transistor on the right side. Byinjecting those impurity ions, a source region 330 and a drain region333 to which P ions are doped in high concentration, thus having N-type,are formed in a manner of self-alignment.

Further, a region 331 to which P ions are doped in low concentration,thus having weak N-type, as well as a channel forming region 332 areformed in the same time. The reason why the region 331 having the weakN-type is formed is because the remaining gate insulating film 328exists. That is, part of P ions transmitting through the gate insulatingfilm 328 is blocked by the gate insulating film 328.

By the same principle, a source region 337 and a drain region 334 havingstrong P-type are formed in a manner of self-alignment and a lowconcentration impurity region 336 is formed in the same time. Further, achannel forming region 335 is formed in the same time. It is noted thatwhen the thickness of the dense anodic oxide films 326 and 327 is asthick as 2000 angstrom, an offset gate region may be formed in contactwith the channel forming region by that thickness.

Its existence may be ignored in the case of the present embodimentbecause the dense anodic oxide films 326 and 327 are so thin as lessthan 1000 angstrom. Then, laser light is irradiated to anneal the regioninto which the impurity ions have been injected. Intense light may bealso irradiated instead of the laser light. Then, a silicon nitride film338 and a silicon oxide film 339 are formed as interlayer insulatingfilms as shown in FIG. 41E. Their thickness is 1000 angstrom,respectively. It is noted that the silicon oxide film 339 needs not beformed in this case.

Here, the thin film transistor is covered by the silicon nitride film.The reliability of the thin film transistor may be enhanced by arrangingas described above because the silicon nitride film is dense and has anexcellent interfacial characteristic. Further, an interlayer insulatingfilm 340 made from a resin material is formed by means of spin coating.Here, the thickness of the interlayer insulating film 340 is 1 μm.

Then, contact holes are created to form a source electrode 341 and adrain electrode 342 of the N-channel type thin film transistor on theleft side. In the same time, a source electrode 343 and the drainelectrode 342 of the thin film transistor on the right side are formed.The electrode 342 is disposed in common to the both of them. Thus, thethin film transistor circuit having a CMOS structure constructed in acomplementary manner may be formed.

In the formation shown in the present embodiment, the thin filmtransistor is covered by the nitride film as well as the resin material.This formation allows to enhance the durability of the thin filmtransistor, so that movable ions nor moisture hardly infiltrate.Further, it allows to prevent capacitance from being generated betweenthe thin film transistor and wires when a multi-layered wire is formed.

[46-th Embodiment]

A 46-th embodiment relates to a case when nickel element is introduceddirectly to the surface of the underlying film in the process shown inthe 39-th embodiment. In this case, the nickel element is held incontact on the lower surface of the amorphous silicon film. In thepresent embodiment, the nickel element is introduced by using acetatenickel aqueous solution after forming the underlying layer to hold thenickel element in contact on the surface of the underlying layer.

Then, a crystal silicon film 267 in which the concentration of nickelhas been reduced is obtained as shown in FIG. 37E by implementing theother processes in the same manner with the case of the 39-thembodiment. Beside the method of using the solution, sputtering, CVD oradsorption may be used as the method for introducing nickel element.Further, the crystal silicon film in which the concentration of metalelement has been reduced can be obtained even when the metal elementwhich promotes crystallization of silicon is used, beside nickel.

[47-th Embodiment]

A 47-th embodiment relates to a case of improving the crystallinity ofan island pattern formed of a crystal silicon film obtained byirradiating laser light in the state shown in FIG. 38E, the state shownin FIG. 39A or the state shown in FIG. 40A. A predetermined annealingeffect can be obtained with relatively low irradiation energy density byirradiating the laser light in the state shown in FIGS. 38E, 39A and40A. This effect is considered to have been effected because the laserenergy is irradiated to a spot of small area, thus enhancing theefficiency of energy utilized in the annealing.

[48-th Embodiment]

A 48-th embodiment relates to a case in which patterning of an activelayer of a thin film transistor is devised in order to enhance theeffect of annealing by the irradiation of laser light. FIGS. 42A through42F show a process for fabricating the thin film transistor according tothe present embodiment. At first, a silicon oxide film or siliconoxynitride film is formed as an underlying layer on a Corning 1737 glasssubstrate 344.

Next, an amorphous silicon film not shown is formed in a thickness of500 angstrom by using the low pressure thermal CVD. This amorphoussilicon film turns out to be a crystal silicon film 346 through thecrystallization process described below. Next, the amorphous siliconfilm not shown is crystallized by the method shown in the 39-th or 41-stembodiments (FIG. 37 or 38) to obtain the crystal silicon film.

Thus, the state shown in FIG. 42A is obtained. The following descriptionwill be made centering on the case of the 39-th embodiment, and the sameapplies to the case of the 41-st embodiment. After obtaining the stateshown in FIG. 42A, the crystal silicon film 346 is formed on the glasssubstrate in accordance to the fabrication process shown in the 39-thembodiment. That is, the amorphous silicon film is crystallized by theheat treatment using nickel element to obtain the crystal silicon film346. The heat treatment is implemented at 620° C. for four hours.

After obtaining the crystal silicon film, a pattern for constructing anactive layer of a thin film transistor is formed. At this time, thepattern is formed so as to have a profile 347 shown in FIG. 42B in orderto suppress the shape of the pattern from being deformed in the latertreatment step of irradiating laser light.

When laser light is irradiated to a pattern 258 made from a normalisland-shape silicon film formed on a base 257 as shown in FIG. 36A, aconvex portion 260 is formed at the edge of a pattern 259 after theirradiation of the laser light as shown in FIG. 36B. It is considered tohappen because energy of the irradiated laser light is concentrated atthe edge of the pattern where heat cannot be released.

The convex portion 260 formed by the above-mentioned phenomenon maybecome a factor of defective wires composing a thin film transistor orof defective operation thereof later. Then, the pattern 347 of theactive layer is formed so as to have the profile as shown in FIG. 42B inthe formation of the present embodiment. Such formation allows tosuppress the pattern of the silicon film from being deformed like theone shown in FIG. 36B when laser light is irradiated. The pattern 347may be realized by utilizing isotropic dry etching and by controllingconditions of the dry etching in patterning it.

It is preferable to set an angle of the part designated by the referencenumeral 348 from 20° to 50°. It is not preferable to set the angle 348below 20° because an area occupied by the active layer increases and itbecomes difficult to form it. Further, it is not also preferable to setthe angle 348 above 50° because the effect for suppressing the shape asshown in FIG. 36B from being formed drops.

After obtaining the pattern (which turns out to be the active layerlater) having the shape 347 in FIG. 42B, laser light is irradiated asshown in FIG. 42C. This step allows to diffuse the nickel element whichis locally gathered within the pattern 347 and to promote thecrystallization of the pattern. After finishing to irradiate laserlight, a heat treatment is implemented within an oxygen atmosphere toform a thermal oxide film 349. The thermal oxide film is formed in 100angstrom thick by implementing the heat treatment for 12 hours in theatmosphere of 100% oxygen at 650° C.

The nickel element contained in the pattern 347 is gettered to thethermal oxide film 349 by the action of oxygen. At this time, becausethe block of the nickel element has been destroyed through theirradiation of laser light in the previous step, the gettering of thenickel element is effectively performed. It is noted that the getteringof nickel element may be performed more effectively if halogen iscontained in the atmosphere for the heat treatment.

Further, the gettering is performed also from the side of the pattern347 when the formation as shown in the present embodiment is adopted.This is useful in enhancing the OFF current characteristics and thereliability of the thin film transistor finally completed. It is becausethe existence of nickel element (or other metal element which promotescrystallization of silicon) existing in the side of the active layerexerts a wide influence over the increase of OFF current and theinstability of the characteristics.

After forming the thermal oxide film 349 for gettering as shown in FIG.42D, it is eliminated. Thus, the state shown in FIG. 42E is obtained. Itis concerned that the silicon oxide film 345 might be etched in the stepof eliminating the thermal oxide film 349 when the silicon oxide film isadopted as the underlying layer 345. However, it does not matter so muchwhen the thickness of the thermal oxide film 349 is as thin as 100angstrom as shown in the present embodiment.

After obtaining the state shown in FIG. 42E, a new thermal oxide film350 is formed in a thickness of 100 angstrom by a heat treatment in anatmosphere of 100% oxygen at 650° C. for four hours. The thermal oxidefilm 350 is effective in suppressing the surface of the pattern 347 frombeing roughened when the laser light is irradiated later. It also formsa part of a gate insulating film later.

Because the thermal oxide film 350 has a very excellent interfacialcharacteristic with the crystal silicon film, it is useful to utilize itas part of the gate insulating film. The laser light may be irradiatedagain after forming the thermal oxide film 350. Thus, the crystalsilicon film 347 in which the concentration of nickel element has beenreduced and which has a high crystallinity may be obtained. Thereafter,the thin film transistor is fabricated by going through the processshown in FIGS. 39 through 41.

[49-th Embodiment]

A 49-th embodiment relates to a case devised in applying a heattreatment at a temperature more than a distortion point of a glasssubstrate. It is preferable to perform the process for gettering themetal element which promotes crystallization of silicon in the presentinvention at a high temperature as much as possible.

When the Corning 1737 glass substrate (distortion point: 667° C.) isused for instance, the higher gettering effect can be obtained when thetemperature in gettering nickel element by forming the thermal oxidefilm is 700° C. rather than when it is 650° C. However, if the heatingtemperature for forming the thermal oxide film is set at 700° C. whileusing the Corning 1737 glass substrate, the glass substrate deforms as aresult.

The present embodiment is to solve this problem. That is, according tothe arrangement shown in the present embodiment, the glass substrate isplaced on a lapping plate which is formed of quartz whose flatness isguaranteed and the heat treatment is implemented in this state. Thereby,the flatness of the softened glass substrate is maintained as well bythe flatness of the lapping plate. It is also important to implementcooling also in the state in which the glass substrate is placed on thelapping plate. The adoption of such arrangement allows the heattreatment to be implemented even if it is in the temperatures more thanthe distortion point of the glass substrate.

[50-th Embodiment]

A 50-th embodiment relates to a case of obtaining a crystal silicon filmon a quartz substrate by utilizing nickel element. In the presentembodiment, an amorphous silicon film formed on the quartz substrate istransformed into the crystal silicon film having a high crystallinity bythe action of nickel element at first.

Next, a thermal oxide film is formed on the crystal silicon film byimplementing a heat treatment within an oxidizing atmosphere to whichHCl is added. At this time, the nickel element remaining in the crystalsilicon film is gettered to the obtained thermal oxide film by theaction of chlorine (Cl). Then, the thermal oxide film containing thenickel element in high concentration as a result of the gettering iseliminated. Thereby, the crystal silicon film in which the concentrationof nickel element is low, while having the high crystallinity isobtained.

FIGS. 43A through 43E are diagrams showing the fabrication processaccording to present embodiment. At first, a silicon oxynitride film 352is formed as an underlying film in a thickness of 5000 angstrom on thequartz substrate 351. It is preferable to form the underlying layer 352to be about 5000 angstrom or more to render it to have a function ofrelaxing a difference of thermal expansion ratio between the quartzsubstrate 351 and a silicon film to be formed later. The siliconoxynitride film is formed by using the plasma CVD using silane, N₂O gasand oxygen as original gases. The plasma CVD using TEOS gas and N₂O gasmay be also used instead of that. It is effective to contain a smallamount of halogen element typified by chlorine in the underlying film352. Thereby, the metal element which promotes crystallization ofsilicon and exists within the semiconductor layer may be gettered by thehalogen element in the later step.

It is also effective to add a hydrogen plasma treatment after formingthe underlying film. It is also effective to implement a plasmatreatment in an atmosphere in which oxygen and hydrogen are mixed. Thesetreatments are effective in eliminating carbon component which isadsorbed on the surface of the underlying film and in enhancing thecharacteristic of interface with a semiconductor film formed later.Next, an amorphous silicon film 353, which turns out to be a crystalsilicon film later, is formed in a thickness of 1500 angstrom by the lowpressure thermal CVD. The reason why the low pressure thermal CVD isused is because thereby, the quality of the crystal silicon filmobtained later is better, i.e. the film quality is denser in concrete.Beside the low pressure thermal CVD, the plasma CVD may be used.

The amorphous silicon film fabricated here is desirable to have 5×10¹⁷cm⁻³ to 2×10¹⁹ cm⁻³ of concentration of oxygen within the film. It isbecause oxygen plays an important role in the later step of getteringthe metal element which promotes crystallization of silicon. However, itmust be careful here because the crystallization of the amorphoussilicon film is hampered if the oxygen concentration is higher than theabove-mentioned range of concentration. The concentration of otherimpurities such as those of nitrogen and carbon is preferred to be lowto the utmost. In concrete, the concentration must be below 2×10¹⁹ cm⁻³.

The thickness of the amorphous silicon film may be selected from a rangefrom about 1000 to 5000 angstrom. Next, nickel element is introduced tothe amorphous silicon film 353 to crystallize it. Here, the nickelelement is introduced by applying nickel acetate solution containing 10ppm (weight conversion) of nickel on the surface of the amorphoussilicon film 353. Beside the method of using the above-mentionedsolution, sputtering, CVD, plasma treatment or adsorption may be used asthe method for introducing the nickel element. Among them, the method ofusing the solution is useful in that it is simple and that theconcentration of the metal element may be readily adjusted.

The nickel acetate solution is applied as described above to form awater film 354 of the nickel acetate solution as shown in FIG. 43A.After obtaining this stage, extra solution is blown out by using a spincoater not shown. Thus, the nickel element is held in contact on thesurface of the amorphous silicon film 353.

It is noted that it is preferable to use nickel sulfate solution forexample, instead of the nickel acetate, if the remained impurities inthe later heating process is taken into consideration. It is because thenickel acetate aqueous solution contains carbon and it might becarbonized in the later heating process, thus remaining within the film.An amount of the nickel element to be introduced may be controlled byadjusting the concentration of nickel salt within the solution.

Next, a heat treatment is implemented in the temperature range from 750°C. to 1100° C. in the state shown in FIG. 43B to crystallize theamorphous silicon film 353 and to obtain a crystal silicon film 355.Here, the heat treatment is implemented within a nitrogen atmosphere(reducing atmosphere) containing 2 volume % of hydrogen at 900° C. forfour hours. The reason why the reducing atmosphere is adopted in thecrystallization step in a way of the heat treatment is to prevent oxidesfrom being created in the step of the heat treatment and moreconcretely, to suppress nickel from reacting with oxygen and NiOx frombeing created on the surface of the film or within the film.

Oxygen couples with nickel and contributes a lot in gettering nickel inthe later gettering step. However, it has been found that if oxygencouples with nickel in the above-mentioned stage of the crystallization,it hampers the crystallization. Accordingly, it is important to suppressthe oxides from being created to the utmost in the crystallization stepin a way of heating.

The concentration of oxygen within the atmosphere for implementing theheat treatment for the crystallization has to be in an order of ppm, orpreferably, less than 1 ppm. Inert gases such as argon, beside nitrogen,or their mixed gas may be used as the gas which occupies the most of theatmosphere for implementing the heat treatment for the crystallization.After obtaining the crystal silicon film 355, it is patterned to form anisland region 356 which turns out to be an active layer of a thin filmtransistor later.

Next, another heat treatment is implemented in a step in FIG. 43D toform a thermal oxide film for gettering the nickel element. This heattreatment is implemented within a nitrogen atmosphere containing 5volume % of oxygen and 3 volume % of HCl to oxygen at 950° C. for oneand half hours. As a result of this step, the thermal oxide film 357 isformed in a thickness of 200 angstrom.

This step is carried out to eliminate the nickel element which has beenintroduced intentionally for the crystallization in the initial stagefrom the crystal silicon film 356 formed in the island pattern. Thisheat treatment is implemented at a temperature higher than that of theheat treatment implemented for the crystallization described above. Itis an important condition for effectively implementing the gettering ofnickel element. It is noted that although this heat treatment may beimplemented in the same or less temperature in the heat treatmentimplemented for the crystallization, it is less effective. It is notedthat the same applies also to the case when another metal element whichpromotes the crystallization of silicon is used.

Because the thermal oxide film 357 is formed, the thickness of thecrystal silicon film 356 formed into the island pattern is reduced toabout 450 angstrom. In the gettering, oxygen existing within the crystalsilicon film plays an important role. That is, the gettering of nickelelement proceeds in a manner in which chlorine acts on nickel oxideformed when oxygen couples with nickel.

If the concentration of oxygen is too much, it becomes the factor ofhampering the crystallization of the amorphous silicon film 353 in thecrystallization step shown in FIG. 43B as described above. However, theexistence thereof plays an important role in the process of getteringnickel as described above. Accordingly, it is important to control theconcentration of oxygen existing within the amorphous silicon film, thestarting film. Because the nickel element is gettered to the oxide filmto be formed in the aforementioned step, naturally the nickelconcentration within the oxide film becomes high as compared to otherregions.

Further, it has been observed that the concentration of nickel elementis apt to be high near the interface between the crystal silicon film356 and the thermal oxide film 357. It is considered to happen becausethe region where the gettering mainly takes place is on the side of theoxide film near the interface between the crystal silicon film and theoxide film. The gettering proceeding near the interface is considered tobe caused by the existence of stress and defects near the interface.

In the present embodiment, the case of using chlorine (Cl) as thehalogen element has been shown and the case of using HCl has been shownas a method for introducing it. However, beside HCl, one type or aplurality of types of mixed gas selected from HF, HBr, Cl₂, F₂, Br₂ maybe used. Beside them, halogen hydride may be used in general. It ispreferable to set the content (volume content) of those gases within theatmosphere to 0.25 to 5% if it is HF, 1 to 15% if it is HBr, 0.25 to 5%if it is Cl₂, 0.125 to 2.5% if it is F₂ and 0.5 to 10% if it is Br₂.

If the concentration is below the above-mentioned range, no significanteffect can be obtained. Further, if the concentration exceeds theabove-mentioned range, the surface of the crystal silicon film isroughened. Then, after forming the thermal oxide film by the heattreatment in the oxidizing atmosphere containing the halogen element, itis eliminated. While the thermal oxide film 357 may be eliminated bymeans of dry etching or wet etching using buffer hydrofluoric acid orother hydrofluorite (hydrofluoric) etchant, it is implemented by the wetetching by using the buffer hydrofluoric acid in the present embodiment.

Thus, the island pattern 356 formed of the crystal silicon film in whichthe concentration of nickel has been reduced is obtained as shown inFIG. 43E. Because nickel element is contained relatively in highconcentration near the surface of the obtained crystal silicon film 356,it is effective to advance the etching of the above-mentioned oxide film357 to over-etch, more or less, the surface of the crystal silicon film356.

It is also effective to irradiate laser light or intense light aftereliminating the thermal oxide film 357 to promote the crystallization ofthe obtained crystal silicon film 356 further. That is, it is effectiveto irradiate the laser light or intense light after gettering nickelelement. KrF excimer laser (wavelength: 248 nm), XeCl excimer laser(wavelength: 308 nm) and other types of excimer lasers may be used asthe laser light to be used. Further, it is possible to irradiateultraviolet ray or infrared ray, i.e. the intense light.

Further, it is effective to form a thermal oxide film not shown byimplementing another heat treatment after obtaining the state shown inFIG. 43E. This thermal oxide film functions as part of a gate insulatingfilm or as the gate insulating film when a thin film transistor isconstructed later. The thermal oxide film has an excellent interfacialcharacteristic with the active layer made from the crystal silicon film,so that it is most suitable as what composes the gate insulating film.

[51-st Embodiment]

A 51-st embodiment relates to a case when Cu is used as the metalelement which promotes crystallization of silicon in the arrangementshown in the 50-th embodiment. In this case, cupric acetate[Cu(CH₃COO)₂] and cupric chloride (CuCl₂2H₂O) may be used as thesolution for introducing Cu. The former is used in the presentembodiment. The state shown in FIG. 43E is obtained by implementingother processes in the same manner with the 50-th embodiment.

[52-nd Embodiment]

A 52-nd embodiment relates to a case of growing crystal in the formdifferent from that in the 50-th embodiment. That is, the presentembodiment relates to a method of growing the crystal in a directionparallel to the substrate, i.e. a method called lateral growth, byutilizing the metal element which promotes crystallization of silicon.

FIGS. 44A through 44E show the fabrication process according to thepresent embodiment. At first, a silicon oxynitride film is formed as anunderlying film 359 in a thickness of 3000 angstrom on a quartzsubstrate 358. Next, an amorphous silicon film 360 which is the startingfilm of a crystal silicon film is formed in a thickness of 2000 angstromby low pressure thermal CVD. It is noted that plasma CVD may be alsoused instead of the low pressure thermal CVD.

Next, a silicon oxide film not shown is formed in a thickness of 1500angstrom and is patterned to form a mask 361. An opening is created onthe mask in a region 362. The amorphous silicon film 360 is exposed atthe region where the opening 362 is created. The opening 362 has a thinand long rectangular shape in the longitudinal direction from the depthto the front side of the figure. Preferably, the width of the opening362 is 20 μm or more. The length thereof in the longitudinal directionmay be determined as necessary.

Then, the nickel acetate aqueous solution containing 10 ppm of nickelelement in terms of weight is applied to the mask 361 and the opening362 and the extra solution is removed by implementing spin drying byusing a spinner not shown. Thus, the solution is held in contact on theexposed surface of the amorphous silicon film 360 as indicated by a dotline 363 in FIG. 44A.

Next, a heat treatment is implemented at 800° C. for four hours in anitrogen atmosphere containing 3 volume % of hydrogen and in whichoxygen is minimized. Then, crystal grows in the direction parallel tothe substrate 358 as indicated by the reference numeral 364 in FIG. 44B.This crystal growth advances from the region of the opening 362 to whichnickel element has been introduced to the surrounding part. This crystalgrowth in the direction parallel to the substrate will be referred to aslateral growth throughout the present specification.

It is possible to advance this lateral growth across more than 100 μmunder the conditions shown in the present embodiment. Then, a siliconfilm having the region in which the crystal has thus grown laterally isobtained. It is noted that crystal growth in the vertical directioncalled vertical growth advances from the surface of the silicon film tothe underlying interface in the region where the opening 362 is formed.

Then, the mask 361 made from the silicon oxide film for selectivelyintroducing nickel element is eliminated. Further, the silicon film ispatterned to form an island pattern 366 made from the crystal siliconfilm in which the crystal has grown in the direction parallel to thesubstrate (i.e. the crystal has grown laterally) as shown in FIG. 44C.

After obtaining the state shown in FIG. 44C, a heat treatment isimplemented at 950° C. for one and half hours within a nitrogenatmosphere containing 10% of oxygen and 3 volume % of HCl to oxygen.FIG. 44D shows this state. In this step, an oxide film 367 containingnickel element in high concentration is formed and thereby, theconcentration of nickel element within the silicon film 366 is reducedrelatively.

Here, the thermal oxide film 367 is formed in a thickness of 200angstrom. The thermal oxide film contains the nickel element getteredwhen it has been formed in high concentration. Further, because thethermal oxide film 367 is formed, the thickness of the crystal siliconfilm 366 is reduced to about 500 angstrom. Next, the thermal oxide film367 containing the nickel element in high concentration is eliminated.Thus, the state shown in FIG. 44E is obtained.

In the active layer (the crystal silicon film formed into the islandshape) 366 of this state, the nickel element has a distribution ofconcentration such that it exists in high concentration toward thesurface of the crystal silicon film. This is caused by the fact that thenickel element has been gettered to the thermal oxide film 367 when thethermal oxide film has been formed. Accordingly, it is useful to etchthe surface of the crystal silicon film further after eliminating thethermal oxide film 367 to eliminate the region in which the nickelelement exists in high concentration.

The concentration of nickel element which remains within the pattern 366made from the laterally grown region thus obtained may be reducedfurther as compared to the case shown in the 50-th embodiment. This iscaused by the fact that the concentration of the metal element containedwithin the laterally grown region is low originally. Thus, theconcentration of nickel element within the pattern 366 made from thelaterally grown region may be readily reduced to the order of 10¹⁷ cm⁻³or less.

When a thin film transistor is formed by utilizing the laterally grownregion of the present embodiment, a semiconductor device having a highermobility may be obtained as compared to the case when the verticallygrown region as shown in the 50-th embodiment (crystal grows verticallyon the whole surface in the case of the 50-th embodiment) is utilized.After obtaining the state shown in FIG. 44E, a thermal oxide film (notshown) is formed on the surface of the active layer 366. This thermaloxide film is formed in a thickness of 500 angstrom by implementing aheat treatment for 30 minutes in an oxygen atmosphere at 950° C.

The thermal oxide film may be formed in a desirable or predeterminedthickness by controlling the heating time, the heating temperature andthe concentration of oxygen within the atmosphere. When the thin filmtransistor is fabricated after that, a silicon oxide film is formed overthe thermal oxide film by plasma CVD or the like to form a gateinsulating film together with the thermal oxide film. Or, the thermaloxide film may be formed in a desirable or predetermined thickness touse it as the gate insulating film as it is.

[53-rd Embodiment]

A 53-rd embodiment relates to a case of fabricating a thin filmtransistor disposed in a pixel region of an active matrix type liquidcrystal display or an active matrix type EL display. FIGS. 45A through45E show the fabrication process according to the present embodiment.

At first, an island-shaped semiconductor layer (made from the crystalsilicon film) patterned into the shape of the active layer is formed onthe glass substrate through the process shown in the 50-th or 52-ndembodiment. The process which follows is implemented in the same mannerwith the both of the embodiments. Next, a thermal oxide film is formedin a thickness of 200 angstrom on the surface thereof by thermaloxidation. In the state shown in FIG. 45A, the reference numeral (369)denotes a glass substrate, (370) an underlying film, and (371) an activelayer formed of the crystal silicon film.

It is noted that before the thermal oxide film 368 is formed, a plasmatreatment is implemented in the reduced pressure atmosphere in whichoxygen and hydrogen are mixed. The plasma is generated by high-frequencydischarge. Organic substances existing on the surface of the exposedactive layer 371 may be removed by the plasma treatment. Specifically,the organic substances adsorbing on the surface of the active layer areoxidized by oxygen plasma and the oxidized organic substances arereduced and vaporized by hydrogen plasma.

Thus the organic substances existing on the surface of the exposedactive layer 371 are removed. The removal of the organic substances isvery effective in suppressing fixed charge from existing on the surfaceof the active layer 371. That is, because the fixed charge caused by theexistence of organic substances hampers the operation of the device andrenders the characteristics thereof instable, it is very useful toremove it.

After obtaining the state shown in FIG. 45A, a silicon oxynitride film372 which composes a gate insulating film is formed in a thickness of1000 angstrom. While the film may be formed by using the plasma CVDusing mixed gas of silane and N₂O and oxygen or the plasma CVD usingmixed gas of TEOS and N₂O, the latter is used in the present embodiment.The silicon oxynitride film 372 functions as the gate insulating filmtogether with the thermal oxide film 368.

When a metal element infiltrates into the gate insulating film, itsfunction as an insulating film drops, thus causing instability anddispersion of the characteristics of the thin film transistor. However,it is effective to contain halogen element within the silicon oxynitridefilm. That is, it is possible to prevent the function of the gateinsulating film as an insulating film from dropping by the influence ofthe nickel element (or another metal element which promotescrystallization of silicon) existing within the active layer by fixingthe nickel element by the action of the halogen element. It issignificant to use the silicon oxynitride film as the insulating film inthat metal element hardly infiltrates to the gate insulating film fromits dense film quality. It is noted that a silicon oxide film which isnormally used may be also used for the gate insulating film.

After forming the silicon oxynitride film 372 which functions as thegate insulating film, an aluminum film not shown which functions as agate electrode later is formed by sputtering. 0.2 weight % of scandiumis contained within the aluminum film to suppress hillock and whiskerfrom being produced in the later process. The hillock and whisker meanthat abnormal growth of aluminum occurs by implementing the heating,thus forming needle or prickle-like projections.

After forming the aluminum film, a dense anodic oxide film not shown isformed. The anodic oxide film is formed by using ethylene glycolsolution containing 3 weight % of tartaric acid as electrolyte. That is,the anodic oxide film having the dense film quality is formed on thesurface of the aluminum film by setting the aluminum film as the anodeand platinum as the cathode and by anodizing within this electrolyte.The thickness of the anodic oxide film not shown having the dense filmquality is around 100 angstrom. This anodic oxide film plays a role ofenhancing the adhesiveness with a resist mask to be formed later. It isnoted that the thickness of the anodic oxide film may be controlled byadjusting voltage applied during the anodization.

Next, the resist mask 374 is formed and the aluminum film is patternedso as to have a pattern 373. The state shown in FIG. 45B is thusobtained. Here, another anodization is implemented. In this case, 3weight % of oxalate aqueous solution is used as electrolyte. A porousanodic oxide film 376 is formed by anodizing within this electrolyte bysetting the aluminum pattern 373 as the anode.

In this step, the anodic oxide film 376 is formed selectively on thesides of the aluminum pattern because the resist mask 374 having thehigh adhesiveness exists thereabove. The anodic oxide film may be grownup to several μm thick. The thickness is 6000 angstrom in the presentembodiment. It is noted that the range of growth may be controlled byadjusting an anodizing time.

Next, the resist mask 306 is removed. Then, a dense anodic oxide film isformed again. That is, the anodization is implemented again by using theaforementioned ethylene glycol solution containing 3 weight % oftartaric acid as electrolyte. Then, an anodic oxide film 377 having adense film quality is formed because the electrolyte infiltrates intothe porous anodic oxide film 376.

This dense anodic oxide film 377 is 1000 angstrom thick. The thicknessis controlled by adjusting applied voltage. Here, the exposed siliconoxynitride film 372 and the thermal oxide film 368 are etched byutilizing dry etching. Then, the porous anodic oxide film 376 iseliminated by using mixed acid in which acetic acid, nitric acid andphosphoric acid are mixed. Thus, the state shown in FIG. 45D isobtained.

After obtaining the state shown in FIG. 45D, impurity ions are injected.Here, P (phosphorus) ions are injected by plasma doping in order tofabricate an N-channel type thin film transistor. In this step, heavilydoped regions 379 and 383 and lightly doped regions 380 and 382 areformed because part of the remaining silicon oxynitride film 378functions as a semi-permeable mask, thus blocking part of the injectedions.

Next, ultraviolet rays are irradiated to activate the regions into whichthe impurity ions have been injected. The irradiation is implemented byusing infrared rays or laser light here. Thus, a source region 379, achannel forming region 381, a drain region 383 and low concentrationimpurity regions 380 and 382 are formed in a manner of self-alignment.The one designated by the reference numeral 382 here is the regioncalled the LDD (lightly doped drain).

It is noted that when the dense anodic oxide film 377 is formed as thickas 2000 angstrom or more, an offset gate region may be formed on theoutside of the channel forming region 381 by its thickness. Although theoffset gate region is formed also in the present embodiment, it is notshown in the figures because its size is small, its contribution due tothe existence thereof is small and the figures might otherwise becomecomplicated.

Next, a silicon oxide film or a silicon nitride film or their laminatedfilm is formed as an interlayer insulating film 384. The silicon oxidefilm is formed here. The interlayer insulating film may be constructedby forming a layer made from a resin material on the silicon oxide filmor the silicon nitride film. Then, contact holes are created to form asource electrode 385 and a drain electrode 386. Thus, the thin filmtransistor shown in FIG. 45E is completed.

[54-th Embodiment]

A 54-th embodiment relates to a case of fabricating a thin filmtransistor through a process different from that shown in FIG. 45. FIGS.46A through 46E show the fabrication process according to the presentembodiment. At first, the crystal silicon film is formed on the glasssubstrate through the process shown in the 50-th or 52-nd embodiment.The process which follows is common to the both embodiments. It is thenpatterned and a plasma treatment is implemented within the reducedpressure atmosphere in which oxygen and hydrogen are mixed.

Then, a thermal oxide film 387 is formed in a thickness of 200 angstrom,thus obtaining the state shown in FIG. 46A. The thermal oxide film 387is formed by implementing a heat treatment for 30 minutes within anoxygen atmosphere at 950° C. In the state shown in FIG. 46A, thereference numeral (388) denotes a glass substrate, (389) an underlyingfilm, (390) an active layer formed of the crystal silicon film. Thethermal oxide film 387 is the film formed again after eliminating thethermal oxide film for gettering.

After obtaining the state shown in FIG. 46A, a silicon oxynitride film391 which composes a gate insulating film is formed in a thickness of1000 angstrom. The film is formed by using the plasma CVD using mixedgas of oxygen, silane and N₂O. It is also possible to use the plasma CVDusing mixed gas of TEOS and N₂O instead of that.

After forming the silicon oxynitride film 391 which functions as thegate insulating film, an aluminum film not shown which functions as agate electrode later is formed by sputtering. 0.2 weight % of scandiumis contained within the aluminum film. After forming the aluminum film,a dense anodic oxide film not shown is formed. The anodic oxide film isformed by using ethylene glycol solution containing 3 weight % oftartaric acid as electrolyte. That is, the anodic oxide film having thedense film quality is formed on the surface of the aluminum film bysetting the aluminum film as the anode and platinum as the cathode andby anodizing within this electrolyte.

The thickness of the anodic oxide film not shown having the dense filmquality is around 100 angstrom. This anodic oxide film plays a role ofenhancing the adhesiveness with a resist mask to be formed later. It isnoted that the thickness of the anodic oxide film may be controlled byadjusting voltage applied during the anodization. Next, the resist mask392 is formed and the aluminum film is patterned so as to have thepattern 393.

Here, another anodization is implemented. In this case, 3 weight % ofoxalate aqueous solution is used as electrolyte. A porous anodic oxidefilm 394 is formed by anodizing within this electrolyte by setting thealuminum pattern 393 as the anode. In this step, the anodic oxide film394 is formed selectively on the sides of the aluminum pattern becausethe resist mask 392 having the high adhesiveness exists thereabove.

The anodic oxide film may be grown up to several μm thick. The thicknessis 6000 angstrom in the present embodiment. It is noted that the rangeof growth may be controlled by adjusting an anodizing time. Next, afterremoving the resist mask 392, a dense anodic oxide film is formed again.That is, the anodization is implemented again by using the ethyleneglycol solution containing 3 weight % of tartaric acid as electrolyte.Then, an anodic oxide film 395 having a dense film quality is formedbecause the electrolyte infiltrates into the porous anodic oxide film394.

Next, the initial injection of impurity ions is implemented. A sourceregion 396 and a drain region 398 are formed by injecting the impurityions. It is noted that no impurity ion is injected to a region 397 atthis time. Next, the porous anodic oxide film 394 is eliminated by usingmixed acid in which acetic acid, nitric acid and phosphoric acid aremixed. Thus, the state shown in FIG. 46D is obtained. After obtainingthe state shown in FIG. 46D, impurity ions are injected again. Theimpurity ions are injected under the doping condition lighter than thatof the first injection.

In this step, lightly doped regions 399 and 400 are formed and a region401 turns out to be a channel forming region. Then, laser light orintense light is irradiated to activate the regions into which theimpurity ions have been injected. The laser light is used here. Thus,the source region 396, the channel forming region 401, the drain region398 and low concentration impurity regions 399 and 400 are formed in amanner of self-alignment. Here, the one designated by the referencenumeral 400 is the region called the LDD (lightly doped drain).

Next, while a silicon oxide film or a silicon nitride film or theirlaminated film is formed as an interlayer insulating film 402, thesilicon nitride film is formed here. The interlayer insulating film maybe also constructed by forming a layer made from a resin material on thesilicon oxide film or the silicon nitride film. After that, contactholes are created to form a source electrode 403 and a drain electrode404. Thus, the thin film transistor shown in FIG. 46E is completed.

[55-th Embodiment]

A 55-th embodiment relates to a case when an N-channel type thin filmtransistor and a P-channel type thin film transistor are formed in acomplementary manner. The formation shown in the present embodiment maybe utilized for various thin film integrated circuits integrated on aninsulating surface as well as for peripheral driving circuits of anactive matrix type liquid crystal display for example. FIGS. 47A through47F are diagrams showing a fabrication process according to the presentembodiment.

At first, a silicon oxide film or a silicon oxynitride film is formed asan underlying film 407 on a glass substrate 406 as shown in FIG. 47A. Itis preferable to use the silicon oxynitride film, and it is used in thepresent embodiment. Next, an amorphous silicon film not shown is formedby the plasma CVD. The low pressure thermal CVD may be also used insteadof that. Then, the amorphous silicon film is transformed into a crystalsilicon film by the method shown in the 50-th embodiment.

Next, a plasma treatment is implemented within an atmosphere in whichoxygen and hydrogen are mixed and the crystal silicon film thus obtainedis patterned to obtain active layers 408 and 409. Thus, the state shownin FIG. 47A is obtained. It is noted that a heat treatment isimplemented at 650° C. for 10 hours within a nitrogen atmospherecontaining 3 volume % of HCl in the state shown in FIG. 47A in order tosuppress the influence of carriers moving the sides of the activelayers.

Because an OFF current characteristic becomes bad if a trap level existsdue to the existence of metal element on the sides of the active layers,it is useful to implement the above-mentioned treatment to lower thedensity of the level on the sides of the active layers. Next, a thermaloxide film 405 and a silicon oxynitride film 410 which compose a gateinsulating film are formed. When quartz is used as the substrate here,it is desirable to compose the gate insulating film only by the thermaloxide film formed by using the above-mentioned thermal oxidation.

Next, an aluminum film not shown which composes a gate electrode lateris formed in a thickness of 4000 angstrom. Beside the aluminum film, ametal which can be anodized, such as tantalum, may be used. Afterforming the aluminum film, a very thin and dense anodic oxide film isformed on the surface thereof by the method described before. Next, aresist mask not shown is placed on the aluminum film to pattern thealuminum film. Then, anodization is implemented by setting the obtainedaluminum pattern as the anode to form porous anodic oxide films 413 and414. The thickness of the porous anodic oxide films is 5000 angstrom.

Then, another anodization is implemented under the condition of formingdense anodic oxide films 415 and 416. The thickness of the dense anodicoxide films 415 and 416 is 800 angstrom. Thus, the state shown in FIG.47B is obtained. Further, the exposed silicon oxynitride film 410 andthe thermal oxide film 405 are eliminated by dry etching, thus obtainingthe state shown in FIG. 47C as a result. After obtaining the state shownin FIG. 47C, the porous anodic oxide films 413 and 414 are eliminated byusing mixed acid in which acetic acid, nitric acid and phosphoric acidare mixed. Thus, the state shown in FIG. 47D is obtained.

Here, resist masks are disposed alternately to inject P (phosphorus)ions to the thin film transistor on the left side and B (boron) ions tothe thin film transistor on the right side. By injecting those impurityions, a source region 419 and a drain region 422 to which P ions aredoped in high concentration, thus having N-type, are formed in a mannerof self-alignment. Further, a region 420 to which P ions are doped inlow concentration, thus having weak N-type, as well as a channel formingregion 421 are formed in the same time.

The reason why the region 420 having the weak N-type is formed isbecause the remaining gate insulating film 417 exists. That is, part ofP ions transmitting through the gate insulating film 417 is blocked bythe gate insulating film 417. By the same principle, a source region 426and a drain region 423 having strong P-type are formed in a manner ofself-alignment and a low concentration impurity region 425 is formed inthe same time. Further, a channel forming region 424 is formed in thesame time.

It is noted that when the thickness of the dense anodic oxide films 415and 416 is as thick as 2000 angstrom, an offset gate region may beformed in contact with the channel forming region by that thickness. Itsexistence may be ignored in the case of the present embodiment becausethe dense anodic oxide films 415 and 416 are so thin as less than 1000angstrom. Then, laser light is irradiated to anneal the region intowhich the impurity ions have been injected. Intense light may be alsoirradiated instead of the laser light.

Then, a silicon nitride film 427 and a silicon oxide film 428 are formedas interlayer insulating films as shown in FIG. 47E. Their thickness is1000 angstrom, respectively. It is noted that the silicon oxide film 428needs not be formed in this case. Thus, the thin film transistor iscovered by the silicon nitride film. The reliability of the thin filmtransistor may be enhanced by arranging as described above because thesilicon nitride film is dense and has an excellent interfacialcharacteristic.

Further, an interlayer insulating film 429 made from a resin material isformed by means of spin coating. Here, the thickness of the interlayerinsulating film 429 is 1 μm. Then, contact holes are created to form asource electrode 430 and a drain electrode 431 of the N-channel typethin film transistor on the left side. In the same time, a sourceelectrode 432 and the drain electrode 431 of the thin film transistor onthe right side are formed. The electrode 431 is disposed in common tothe both of them. Thus, the thin film transistor circuit in which theN-channel type thin film transistor and the P-channel type thin filmtransistor are constructed in a complementary manner as shown in FIG.47F is completed.

As described above, the thin film transistor having the CMOS structureconstructed in the complementary manner may be formed. In the formationshown in the present embodiment, the thin film transistor may be coveredby the nitride film as well as the resin material. This formation allowsto enhance the durability of the thin film transistor, so that movableions nor moisture hardly infiltrate. Further, it allows to preventcapacitance from being generated between the thin film transistor andwires when a multi-layered wire is formed.

[56-th Embodiment]

A 56-th embodiment relates to a case when nickel element is introduceddirectly to the surface of the underlying film in the process shown inthe 50-th embodiment. In this case, the nickel element is held incontact on the lower surface of the amorphous silicon film. In thiscase, the nickel element is introduced after forming the underlyinglayer to hold the nickel element (metal element) in contact on thesurface of the underlying layer.

According to the present embodiment, aqueous solution of nickel acetateis applied on the surface of the underlying film to introduce nickelelement directly to it and the other processes are implemented in thesame manner with the case of the 50-th embodiment to obtain an islandshape pattern 356 formed of a crystal silicon film in which theconcentration of nickel has been reduced as shown in FIG. 43E. Besidethe method of using the solution, sputtering, CVD or adsorption may beused as the method for introducing nickel element.

[57-th Embodiment]

A 57-th embodiment relates to a case of improving the crystallinity ofan island pattern formed of an obtained crystal silicon film byirradiating laser light in the state shown in FIG. 43E, the state shownin FIG. 44E, the state shown in FIG. 45A or the state shown in FIG. 46A.

A predetermined annealing effect can be obtained with relatively lowirradiation energy density by irradiating the laser light in the stateshown in FIGS. 43E, 44E, 45A and 46A as compared to the case ofimplementing annealing to the whole film before the patterning. Thiseffect is considered to have been effected because the laser energy isirradiated to a spot of small area, thus enhancing the efficiency ofenergy utilized in the annealing.

[58-th Embodiment]

A 58-th embodiment relates to a case of obtaining a crystal silicon filmon a glass substrate by utilizing nickel element. In the presentembodiment, the crystal silicon film having a high crystallinity isobtained by the action of nickel element at first. Then, laser light isirradiated to enhance the crystallinity of the film and to diffuse thenickel element which is concentrated locally within the film, i.e. toreduce or to extinguish blocks of nickel.

Next, an oxide film containing F (fluorine) is formed on the crystalsilicon film by thermal oxidation. At this time, the nickel elementremaining in the crystal silicon film is gettered to the thermal oxidefilm by the action of F element. At this time, because the nickelelement is dispersed therein by the irradiation of laser light, thegettering proceeds effectively. Then, the thermal oxide film containingthe nickel element in high concentration as a result of the gettering iseliminated. Thereby, the crystal silicon film in which the concentrationof nickel element is low, while having the high crystallinity, isobtained on the glass substrate.

The fabrication process of the present embodiment will be explained withreference to FIGS. 48A through 48E. At first, a silicon oxide film 434is formed as an underlying film in a thickness of 3000 angstrom on theglass substrate 433 of Corning 1737 (distortion point: 667° C.). Thisfilm is formed by using sputtering. The silicon oxide film 434 has afunction of suppressing the diffusion of impurities from the glasssubstrate in the later steps. It also has a function of relaxing stressacting the glass substrate and a silicon film to be formed later.

It is also effective to contain a small amount of halogen element in theunderlying film 434. Thereby, the metal element which promotescrystallization of silicon which exists within the semiconductor layermay be gettered to the underlying film by the halogen element in thelater step. It is also effective to add a hydrogen plasma treatmentafter forming the underlying film. Because it is effective ineliminating carbon component which exists on the surface of theunderlying film and in suppressing a level of fixed charge from existingon the interface with the silicon film to be formed later. It is alsoeffective to implement a plasma treatment in an atmosphere in whichoxygen and hydrogen are mixed.

Next, an amorphous silicon film 435, which turns out to be a crystalsilicon film later, is formed in a thickness of 500 angstrom by the lowpressure thermal CVD. The reason why the low pressure thermal CVD isused is because thereby, the quality of the crystal silicon filmobtained later is better, i.e. the film quality is denser in concrete.Beside the low pressure thermal CVD, the plasma CVD may be used. Theamorphous silicon film fabricated here is desirable to have 5×10¹⁷ cm⁻³to 2×10¹⁹ cm⁻³ of oxygen concentration within the film.

It is because oxygen plays an important role in the later step ofgettering the metal element which promotes crystallization of silicon(in the step of gettering nickel in case of the present embodiment).However, it must be careful here because the crystallization of theamorphous silicon film is hampered if the oxygen concentration is higherthan the above-mentioned range of concentration. When the oxygenconcentration is lower than the above-mentioned concentration range onthe other hand, it contributes less to the action for gettering themetal element. The concentration of other impurities such as those ofnitrogen and carbon is preferred to be low to the utmost. In concrete,their concentration is preferable to be below 2×10¹⁹ cm⁻³.

The upper limit of the thickness of this amorphous silicon film is about2000 angstrom. It is because a thick film is disadvantageous inobtaining the effect of the irradiation of laser light implementedlater. That is, the most of laser light irradiated to the silicon filmis absorbed by the surface of the film. The lower limit of the thicknessof the amorphous silicon film 435 is practically about 200 angstrom,though it depends on a method for forming the film. If the thickness isbelow that, there will be a problem in the uniformity of the film.

Next, metal element is introduced to the amorphous silicon film 435 tocrystallize it. Here, the nickel element is used as the metal elementwhich promotes crystallization of silicon. Here, a method of usingsolution is utilized as a method for introducing nickel element. Here,the nickel element is introduced by applying nickel acetate solutioncontaining 10 ppm (weight conversion) of nickel on the surface of theamorphous silicon film 435. Beside the method of using theabove-mentioned solution, sputtering, CVD, plasma treatment oradsorption may be used as the method for introducing the nickel element.Among them, the method of using the solution is useful in that it issimple and that the concentration of the metal element may be readilyadjusted.

The nickel acetate solution is applied as described above to form awater film 436 of the nickel acetate aqueous solution as shown in FIG.48A. After obtaining this state, extra solution is blown out by using aspinner not shown. Thus, the nickel element is held in contact on thesurface of the amorphous silicon film 435. An amount of the nickelelement to be introduced to the amorphous silicon film 435 may becontrolled also by adjusting the time for holding the water film 436 orthe condition for removing it by means of the spinner.

It is noted that it is preferable to use nickel sulfate solution,instead of the nickel acetate, if the remained impurities in the laterheating process is taken into consideration. It is because the nickelacetate aqueous solution contains carbon and it might be carbonized inthe later heating process, thus remaining within the film.

Next, a heat treatment is implemented within a nitrogen atmospherecontaining 3 volume % of hydrogen at the temperature 640° C. in thestate shown in FIG. 48B to crystallize the amorphous silicon film 435and to obtain a crystal silicon film 437. The heating time is fourhours. While the heat treatment may be implemented in the temperaturerange of 500 to 700° C., it is preferable to be a temperature below thedistortion point of the glass substrate. Because the distortion point ofthe Corning 1737 glass substrate used in the present embodiment is 667°C., it is preferable to set the upper limit of the heating temperaturehere at about 650° C., leaving some margin.

The reason why the reducing atmosphere is adopted in the crystallizationstep in a way of the heat treatment is to prevent oxides from beingcreated in the step of the heat treatment and more concretely, tosuppress nickel from reacting with oxygen and NiOx from being created onthe surface of the film or within the film. Oxygen couples with nickeland contributes a lot in gettering nickel in the later gettering step.However, it has been found that if oxygen couples with nickel in theabove-mentioned stage of the crystallization, it hampers thecrystallization. Accordingly, it is important to suppress the oxidesfrom being created to the utmost in the crystallization step in a way ofheating.

The concentration of oxygen within the atmosphere for implementing theheat treatment for the crystallization has to be in the order of ppm, orpreferably, less than 1 ppm. Inert gases such as argon, beside nitrogen,or their mixed gas may be used as the gas which occupies the most of theatmosphere for implementing the heat treatment for the crystallization.After the crystallization step by the heat treatment described above,nickel element remains in certain blocks. It has been confirmed by anobservation through a TEM (Transmission type Electron Microscope). Whilethe reason why nickel exists in certain blocks has not been clarifiedyet, it is considered to be related with some crystallization mechanism.

Next, laser light is irradiated as shown in FIG. 48C. Here, KrF excimerlaser (wavelength: 248 nm) is used here. The irradiation is implementedby scanning the laser beam whose shape is linear. The irradiation of thelaser light allows the nickel element which has been concentratedlocally as a result of the crystallization by means of theabove-mentioned heat treatment to be distributed within the film 437 insome degree. That is, it allows to extinguish or reduce the blocks ofnickel element and to distribute the nickel element. For the laser lightdescribed above, other types of excimer laser such as XeCl excimer laser(wavelength: 308 nm) may be used. Further, beside laser light, it ispossible to arrange so as to irradiate ultraviolet rays or infrared raysfor example.

Next, another heat treatment is implemented in a step in FIG. 48D toform a thermal oxide film for gettering the nickel element. This heattreatment is implemented within an oxygen atmosphere containing 3 volume% of hydrogen and 100 ppm of ClF₃ at 640° C. As a result of this step,the thermal oxide film is formed in a thickness of 200 angstrom.

This step is carried out to eliminate the nickel element which has beenintroduced intentionally for the crystallization in the initial stagefrom the crystal silicon film 437. This heat treatment is implemented inthe temperature range of 500 to 700° C. when a normal glass substrate isused as the substrate. The upper limit of the heating temperature islimited by the distortion point of the glass substrate to be used and itmust be careful not to implement the heat treatment in temperaturesabove the distortion point of the glass substrate because otherwise thesubstrate deforms.

In this step, because the nickel element which has been distributed bythe irradiation of laser light described above is gettered to the oxidefilm 438 to be formed, naturally the nickel concentration within theoxide film 438 becomes high as compared to other regions. Further, ithas been observed that the concentration of nickel element is apt to behigh near the interface between the crystal silicon film 437 and thethermal oxide film 438. It is considered to happen because the regionwhere the gettering mainly takes place is on the side of the oxide filmnear the interface between the crystal silicon film and the oxide film.

The gettering proceeding near the interface is considered to be causedby the existence of stress and defects near the interface. It is alsoobserved that the concentration of fluorine and chlorine is higher nearthe interface between the silicon film 437 and the thermal oxide film438. The crystal silicon film thus obtained contains the metal elementwhich promotes the crystallization of silicon in concentration of 1×10¹⁶cm⁻³ to 5×10¹⁸ cm⁻³, fluorine atoms in concentration of 1×10¹⁵ cm⁻³ to1×10²⁰ cm⁻³, and hydrogen atoms in concentration of 1×10¹⁷ cm⁻³ to1×10²¹ cm⁻³.

Then, after finishing to form the thermal oxide film 438 containingnickel in high concentration and shown in FIG. 48D, it is eliminated.While the thermal oxide film 438 may be eliminated by means of dryetching or wet etching using buffer hydrofluoric acid or otherhydrofluorite (hydrofluoric) etchant, it is implemented by using thebuffer hydrofluoric acid in the present embodiment. Thus, a crystalsilicon film 439 in which the concentration of nickel has been reducedis obtained as shown in FIG. 48E.

Because nickel element is contained near the surface of the obtainedcrystal silicon film 439 relatively in high concentration, it iseffective to advance the etching of the above-mentioned oxide film 438to over-etch, more or less, the surface of the crystal silicon film 439.It is also effective to irradiate laser light again after eliminatingthe thermal oxide film 438 to promote the crystallization of theobtained crystal silicon film 439 further.

[59-th Embodiment]

A 59-th embodiment relates to a case when Cu is used as the metalelement which promotes crystallization of silicon in the arrangementshown in the 58-th embodiment. In this case, while cupric acetate[Cu(CH₃COO)₂] and cupric chloride (CuCl₂2H₂O) may be used as thesolution for introducing Cu, cupric chloride (CuCl₂2H₂O) is used here.The other processes are implemented in the same manner with the 58-thembodiment.

[60-th Embodiment]

A 60-th embodiment relates to a case of growing crystal in the formdifferent from that in the 58-th embodiment. That is, the presentembodiment relates to a method of growing the crystal in a directionparallel to the substrate, i.e. a method called lateral growth, byutilizing the metal element which promotes crystallization of silicon.

FIGS. 49A through 49E show the fabrication process according to thepresent embodiment.

At first, a silicon oxynitride film is formed as an underlying film 441in a thickness of 3000 angstrom on a quartz substrate 440. Next, anamorphous silicon film 442 which is the starting film of a crystalsilicon film is formed in a thickness of 600 angstrom by low pressurethermal CVD. The thickness of this amorphous silicon film is preferableto be less than 2000 angstrom as described before. Other substrates suchas a quartz substrate may be used as the above-mentioned substrate.

Next, a silicon oxide film not shown is formed in a thickness of 1500angstrom and is patterned to form a mask 443. An opening is created onthe mask in a region 444. The amorphous silicon film 442 is exposed atthe region where the opening 444 is created. The opening 444 has a thinand long rectangular shape in the longitudinal direction from the depthto the front side of the figure. Preferably, the width of the opening444 is 20 μm or more. The length thereof in the longitudinal directionmay be determined as necessary.

Then, the nickel acetate aqueous solution containing 10 ppm of nickelelement in terms of weight is applied and the extra solution is removedby implementing spin drying by using a spinner. Thus, the solution isheld in contact on the exposed surface of the amorphous silicon film 442as indicated by a dot line 445 in FIG. 49A.

Next, a heat treatment is implemented at 640° C. for four hours in anitrogen atmosphere containing 3 volume % of hydrogen and in whichoxygen is minimized. Then, crystal grows in the direction parallel tothe substrate 440 as indicated by the reference numeral 446 in FIG. 49B.This crystal growth advances from the region of the opening 444 to whichnickel element has been introduced to the surrounding part. This crystalgrowth in the direction parallel to the substrate will be referred to aslateral growth throughout the present specification.

It is possible to advance this lateral growth across more than 100 μmunder the conditions shown in the present embodiment. Then, a siliconfilm 447 having the region in which the crystal has thus grown laterallyis obtained. It is noted that crystal growth in the vertical directioncalled vertical growth advances from the surface of the silicon film tothe underlying interface in the region where the opening 444 is formed.Then, the mask 443 made from the silicon oxide film for selectivelyintroducing nickel element is eliminated. In this state, the verticallygrown region, the laterally grown region and the region where no crystalhas grown (thus having an amorphous state) exist within the silicon film447.

It this state, nickel element is unevenly distributed within the film.In particular, the nickel element exists relatively in highconcentration at the region where the opening 444 is created and theedge portion of the crystal growth indicated by the reference numeral446. Next, after obtaining the state shown in FIG. 49C, laser light isirradiated in the same manner with the 58-th embodiment by using the KrFexcimer laser. This step allows to distribute the unevenly distributednickel element and to create the circumstance in which the gettering canbe readily implemented in the later gettering step.

After irradiating the laser light, a heat treatment is implemented at650° C. within an atmosphere containing 3 volume % of hydrogen and 100ppm of NF₃. In this step, an oxide film 448 containing the nickelelement in high concentration is formed in a thickness of 200 angstromand thereby, the concentration of the nickel element within the siliconfilm 447 is reduced relatively in the same time. Then, after forming thethermal oxide film 448 containing the nickel element in highconcentration, it is eliminated.

It is useful to etch the surface of the crystal silicon film furtherafter eliminating the thermal oxide film 448. Then, patterning isimplemented to create a pattern 449 made from the laterally grownregion. The concentration of nickel element which remains within thepattern 449 made from the laterally grown region thus obtained may bereduced further as compared to the case shown in the 58-th embodiment.

This is caused by the fact that the concentration of the metal elementcontained within the laterally grown region is low originally. Inconcrete, the concentration of nickel element within the pattern 449made from the laterally grown region may be readily reduced to the orderof 10¹⁷ cm⁻³ or less. It is also useful to implement the etchingprocesses further after forming the pattern shown in FIG. 49E toeliminate the nickel element existing on the surface of the pattern.

Then, a thermal oxide film 450 is formed on the pattern 449 thus formed.The thermal oxide film is formed in a thickness of 200 angstrom byimplementing a heat treatment for 12 hours in an oxygen atmosphere at650° C. It is also effective to contain fluorine within the atmospherein forming the thermal oxide film 450. Thereby, the nickel element maybe fixed and unpaired bonding hands (dangling bonds) on the surface ofthe silicon film may be neutralized. That is, the interfacialcharacteristics between the active layer and the gate insulating filmmay be improved.

Chlorine may be used instead of fluorine. It is noted that the thermaloxide film becomes a part of the gate insulating film later when a thinfilm transistor is formed. When the thin film transistor is fabricatedafter that, the gate insulating film is formed by forming a siliconoxide film covering the thermal oxide film 456 by means of plasma CVD orthe like.

[61-st Embodiment]

A 61-st embodiment relates to a case of fabricating a thin filmtransistor disposed in a pixel region of an active matrix type liquidcrystal display or an active matrix type EL display.

FIGS. 50A through 50E show the fabrication process according to thepresent embodiment. At first, the crystal silicon film is formed on theglass substrate through the process shown in the 58-th or 60-thembodiment. When the crystal silicon film is obtained through thearrangement shown in the 58-th embodiment, it is patterned and the stateshown in FIG. 50A is obtained after going through the steps shown inFIG. 48. The process which follows is common to the both embodiments.

In the state shown in FIG. 50A, the reference numeral (452) denotes aglass substrate, (453) an underlying film, (454) an active layer formedof the crystal silicon film. After obtaining the state shown in FIG.50A, a plasma treatment is implemented in the reduced pressureatmosphere in which oxygen and hydrogen are mixed. The plasma isgenerated by high-frequency discharge. Organic substances existing onthe surface of the exposed active layer 454 may be removed by the plasmatreatment. Specifically, the organic substances adsorbing on the surfaceof the active layer are oxidized by oxygen plasma and the oxidizedorganic substances are reduced and vaporized by hydrogen plasma. Thusthe organic substances existing on the surface of the exposed activelayer 454 are removed.

The removal of the organic substances is very effective in suppressingfixed charge from existing on the surface of the active layer 454. Thatis, because the fixed charge caused by the existence of organicsubstances hampers the operation of the device and renders thecharacteristics thereof instable, it is very useful to remove it. Afterremoving the organic substances, a thermal oxidation is implementedwithin an oxygen atmosphere at 640° C. to form a thermal oxide film of100 angstrom thick. This thermal oxide film has a high interfacialcharacteristic with a semiconductor layer and composes a part of a gateinsulating film later. Thus, the state shown in FIG. 50A is obtained.

After obtaining the state shown in FIG. 50A, a silicon oxide film 455which composes the gate insulating film is formed in a thickness of 1000angstrom. The film is formed by using the plasma CVD. The silicon oxidefilm 455 functions as the gate insulating film together with the thermaloxide film 451. It is effective to contain halogen element within thesilicon oxide film 455. In this case, it is possible to prevent thefunction of the gate insulating film as an insulating film from droppingby the influence of the nickel element (or another metal element whichpromotes crystallization of silicon) existing within the active layer byfixing the nickel element by the action of the halogen element.

After forming the silicon oxide film 455 which functions as the gateinsulating film, an aluminum film not shown which functions as a gateelectrode later is formed by sputtering. 0.2 weight % of scandium iscontained within the aluminum film to suppress hillock and whisker frombeing produced in the later process. The hillock and whisker mean thatabnormal growth of aluminum occurs by implementing the heating, thusforming needle or prickle-like projections.

After forming the aluminum film, a dense anodic oxide film not shown isformed. The anodic oxide film is formed by using ethylene glycolsolution containing 3 weight % of tartaric acid as electrolyte. That is,the anodic oxide film having the dense film quality is formed on thesurface of the aluminum film by setting the aluminum film as the anodeand platinum as the cathode and by anodizing within this electrolyte.The thickness of the anodic oxide film not shown having the dense filmquality is around 100 angstrom. This anodic oxide film plays a role ofenhancing the adhesiveness with a resist mask to be formed later. It isnoted that the thickness of the anodic oxide film may be controlled byadjusting voltage applied during the anodization.

Next, the resist mask 457 is formed and the aluminum film is patternedso as to have a pattern 456. The state shown in FIG. 50B is thusobtained. Here, another anodization is implemented. In this case, 3weight % of oxalate aqueous solution is used as electrolyte. A porousanodic oxide film 459 is formed by anodizing within this electrolyte bysetting the aluminum pattern 456 as the anode.

In this step, the anodic oxide film 459 is formed selectively on thesides of the aluminum pattern because the resist mask 457 having thehigh adhesiveness exists thereabove. The anodic oxide film 459 may begrown up to several μm thick. The thickness is 6000 angstrom here. It isnoted that the range of growth may be controlled by adjusting ananodizing time.

Next, after removing the resist mask 457, a dense anodic oxide film isformed again. That is, the anodization is implemented again by using theaforementioned ethylene glycol solution containing 3 weight % oftartaric acid as electrolyte. Then, an anodic oxide film 460 having adense film quality is formed because the electrolyte infiltrates intothe porous anodic oxide film 459.

This dense anodic oxide film 460 is 1000 angstrom thick. The thicknessis controlled by adjusting applied voltage. Then, the exposed siliconoxide film 455 and the thermal oxide film 451 are etched by utilizingdry etching. Then, the porous anodic oxide film 459 is eliminated byusing mixed acid in which acetic acid, nitric acid and phosphoric acidare mixed. Thus, the state shown in FIG. 50D is obtained.

After obtaining the state shown in FIG. 50D, impurity ions are injected.Here, P (phosphorus) ions are injected by plasma doping in order tofabricate an N-channel type thin film transistor. In this step, heavilydoped regions 462 and 466 and lightly doped regions 463 and 465 areformed because part of the remaining silicon oxide film 461 functions asa semi-permeable mask, thus blocking part of the injected ions.

Next, laser light is irradiated to activate the regions into which theimpurity ions have been injected. Intense light may be applied insteadof the laser light. Thus, a source region 462, a channel forming region464, a drain region 466 and low concentration impurity regions 463 and465 are formed in a manner of self-alignment. The one designated by thereference numeral 465 here is the region called the LDD (lightly dopeddrain).

It is noted that when the dense anodic oxide film 460 is formed as thickas 2000 angstrom or more, an offset gate region may be formed on theoutside of the channel forming region 464 by its thickness. Although theoffset gate region is formed also in the present embodiment, it is notshown in the figures because its size is small, its contribution due tothe existence thereof is small and the figures might otherwise becomecomplicated. It is noted that it must be careful in forming the anodicoxide film having the dense film quality as thick as 2000 angstrom ormore because it requires more than 200 V of applied voltage.

Next, a silicon oxide film or a silicon nitride film or their laminatedfilm is formed as an interlayer insulating film 467. Their laminatedfilm is formed here. The interlayer insulating film may be constructedby forming a layer made from a resin material on the silicon oxide filmor the silicon nitride film. Then, contact holes are created to form asource electrode 468 and a drain electrode 469. Thus, the thin filmtransistor shown in FIG. 50E is obtained.

[62-nd Embodiment]

A 62-nd embodiment relates to a case of fabricating a thin filmtransistor through a process different from that shown in FIG. 50 (the61-st embodiment). FIGS. 51A through 51E show the fabrication processaccording to the present embodiment. At first, the crystal silicon filmis formed on the glass substrate through the process shown in the 58-thor 60-th embodiment. It is then patterned to obtained the state shown inFIG. 51A. The process which follows is common to the both embodiments.

After obtaining the state shown in FIG. 51A, a plasma treatment isimplemented within the reduced pressure atmosphere in which oxygen andhydrogen are mixed. In the state shown in FIG. 51A, the referencenumeral (471) denotes a glass substrate, (472) an underlying film madefrom a silicon oxide film, (473) an active layer formed of the crystalsilicon film. The thermal oxide film 470 is the film formed again aftereliminating the thermal oxide film for gettering. Then, a silicon oxidefilm 474 which composes a gate insulating film is formed in a thicknessof 1000 angstrom by using the plasma CVD as shown in FIG. 51B.

The silicon oxide film 474 composes the gate insulating film togetherwith the thermal oxide film 470. Next, an aluminum film not shown whichfunctions as a gate electrode later is formed by sputtering. 0.2 weight% of scandium is contained within the aluminum film. After forming thealuminum film, a dense anodic oxide film not shown is formed. The anodicoxide film is formed by using ethylene glycol solution containing 3weight % of tartaric acid as electrolyte.

The thickness of the anodic oxide film not shown having the dense filmquality is around 100 angstrom. This anodic oxide film plays a role ofenhancing the adhesiveness with a resist mask to be formed later. It isnoted that the thickness of the anodic oxide film may be controlled byadjusting voltage applied during the anodization. Next, the resist mask475 is formed and the aluminum film is patterned so as to have thepattern 476.

Here, another anodization is implemented. In this case, 3 weight % ofoxalate aqueous solution is used as electrolyte. A porous anodic oxidefilm 477 is formed by anodizing within this electrolyte by setting thealuminum pattern 476 as the anode. In this step, the anodic oxide film477 is formed selectively on the sides of the aluminum pattern becausethe resist mask 475 having the high adhesiveness exists thereabove.

The anodic oxide film may be grown up to several μm thick. The thicknessis 6000 angstrom here. It is noted that the range of growth may becontrolled by adjusting an anodizing time. Next, after removing theresist mask 475, a dense anodic oxide film is formed again. That is, theanodization is implemented again by using the ethylene glycol solutioncontaining 3 weight % of tartaric acid as electrolyte. Then, an anodicoxide film 478 having a dense film quality is formed because theelectrolyte infiltrates into the porous anodic oxide film 477.

Here, the initial injection of impurity ions is implemented. Here, P(phosphorus) ions are injected in order to fabricate an N-channel typethin film transistor. It is noted that B (boron) ions are injected whena P-channel type thin film transistor is to be fabricated. A sourceregion 479 and a drain region 481 are formed by injecting the impurityions. It is noted that no impurity ion is injected to a region 480 atthis time. Next, the porous anodic oxide film 477 is eliminated by usingmixed acid in which acetic acid, nitric acid and phosphoric acid aremixed. Thus, the state shown in FIG. 51D is obtained.

After obtaining the state shown in FIG. 51D, impurity ions are injectedagain. The impurity ions are injected under the doping condition (lowerdosage) lighter than that of the first injection. In this step, lightlydoped regions 482 and 483 are formed and a region 484 turns out to be achannel forming region.

Then, laser light is irradiated to activate the regions into which theimpurity ions have been injected. Instead of the laser light, infraredrays or ultraviolet rays may be also irradiated. Thus, the source region479, the channel forming region 484, the drain region 481 and lowconcentration impurity regions 482 and 483 are formed in a manner ofself-alignment. Here, the one designated by the reference numeral 483 isthe region called the LDD (lightly doped drain).

Next, while a silicon oxide film or a silicon nitride film or theirlaminated film is formed as an interlayer insulating film 485, it isformed by using the silicon oxide film here. The interlayer insulatingfilm may be also constructed by forming a layer made from a resinmaterial on the silicon oxide film or the silicon nitride film. Afterthat, contact holes are created to form a source electrode 486 and adrain electrode 487. Finally, a heat treatment (hydrogenation heattreatment) is implemented for one hour within a hydrogen atmosphere at350° C. In this step, defects and unpaired bonding hands (danglingbonds) may be neutralized. Thus, the thin film transistor shown in FIG.51E is obtained.

[63-rd Embodiment]

A 63-rd embodiment relates to a case when an N-channel type thin filmtransistor and a P-channel type thin film transistor are formed in acomplementary manner. The formation shown in the present embodiment maybe utilized for various thin film integrated circuits integrated on aninsulating surface as well as for peripheral driving circuits of anactive matrix type liquid crystal display for example. FIGS. 52A through52F are diagrams showing a fabrication process according to the presentembodiment.

At first, a silicon oxide film is formed as an underlying FIG. 490 on aglass substrate 489 as shown in FIG. 52A. It is also possible to use asilicon nitride film instead of the silicon oxide film. Next, anamorphous silicon film not shown is formed by the plasma CVD or the lowpressure thermal CVD. The former is used here. Then, the amorphoussilicon film is transformed into a crystal silicon film by the methodshown in the 58-th embodiment. Next, a plasma treatment is implementedwithin an atmosphere in which oxygen and hydrogen are mixed and thecrystal silicon film thus obtained is patterned to obtain active layers491 and 492. After forming the active layers 491 and 492, a thermaloxide film 488 is formed. The thickness of the thermal oxide film 488 isaround 100 angstrom. Thus, the state shown in FIG. 52A is obtained.

Next, a silicon oxide film which compose a gate insulating film 493 isformed. Next, an aluminum film not shown is formed in a thickness of4000 angstrom. Beside the aluminum film, a metal which can be anodized,such as tantalum, may be used. After forming the aluminum film, a verythin and dense anodic oxide film is formed on the surface thereof by themethod described before.

Next, a resist mask not shown is placed on the aluminum film to patternthe aluminum film. Then, anodization is implemented by setting theobtained aluminum pattern as the anode to form porous anodic oxide films496 and 497. The thickness of the porous anodic oxide films is 5000angstrom. Then, another anodization is implemented under the conditionof forming dense anodic oxide films 498 and 499.

The thickness of the dense anodic oxide films 498 and 499 is 800angstrom. Thus, the state shown in FIG. 52B is obtained. Further, theexposed silicon oxide film 493 and the thermal oxide film 488 areeliminated by dry etching, thus obtaining the state shown, in FIG. 52Cas a result. After obtaining the state shown in FIG. 52C, the porousanodic oxide films 496 and 497 are eliminated by using mixed acid inwhich acetic acid, nitric acid and phosphoric acid are mixed. Thus, thestate shown in FIG. 52D is obtained.

Here, resist masks are disposed alternately to inject P (phosphorus)ions to the thin film transistor on the left side and B (boron) ions tothe thin film transistor on the right side. By injecting those impurityions, a source region 502 and a drain region 505 to which P ions aredoped in high concentration, thus having N-type, are formed in a mannerof self-alignment. Further, a region 503 to which P ions are doped inlow concentration, thus having weak N-type, as well as a channel formingregion 504 are formed in the same time.

The reason why the region 503 having the weak N-type is formed isbecause the remaining gate insulating film 500 exists. That is, part ofP ions transmitting through the gate insulating film 500 is blocked bythe gate insulating film 500. By the same principle, a source region 509and a drain region 506 having strong P-type are formed in a manner ofself-alignment and a low concentration impurity region 508 is formed inthe same time. Further, a channel forming region 507 is formed in thesame time.

It is noted that when the thickness of the dense anodic oxide films 498and 499 is as thick as 2000 angstrom, an offset gate region may beformed in contact with the channel forming region by that thickness. Itsexistence may be ignored in the case of the present embodiment becausethe dense anodic oxide films 498 and 499 are so thin as less than 1000angstrom.

Then, laser light is irradiated to anneal the region into which theimpurity ions have been injected. Intense light such as ultraviolet raysmay be also irradiated instead of the laser light. Then, a siliconnitride film 510 and a silicon oxide film 511 are formed as interlayerinsulating films as shown in FIG. 52E. Their thickness is 1000 angstrom,respectively. It is noted that the silicon oxide film 511 needs not bealways formed. Thus, the thin film transistor is covered by the siliconnitride film. The reliability of the thin film transistor may beenhanced by arranging as described above because the silicon nitridefilm is dense and has an excellent interfacial characteristic.

Further, an interlayer insulating film 512 made from a resin material isformed by means of spin coating. Here, the thickness of the interlayerinsulating film 512 is 1 μm. Then, contact holes are created to form asource electrode 513 and a drain electrode 514 of the N-channel typethin film transistor on the left side. In the same time, a sourceelectrode 515 and the drain electrode 514 of the thin film transistor onthe right side are formed. The electrode 514 is disposed in common tothe both of them.

Thus, the thin film transistor circuit having the CMOS structureconstructed in the complementary manner as shown in FIG. 52F iscompleted. In the formation shown in the present embodiment, the thinfilm transistor may be covered by the nitride film as well as the resinmaterial. This formation allows to enhance the durability of the thinfilm transistor, so that movable ions nor moisture hardly infiltrate.Further, it allows to prevent capacitance from being generated betweenthe thin film transistor and wires when a multi-layered wire is formed.

[64-th Embodiment]

A 64-th embodiment relates to a case when nickel element is introduceddirectly to the surface of the underlying film in the process shown inthe 58-th embodiment in this case, the nickel element is held in contacton the lower surface of the amorphous silicon film. In this case, thenickel element is introduced after forming the underlying layer to holdthe nickel element (metal element) in contact on the surface of theunderlying layer.

According to the present embodiment, aqueous solution of nickel acetateis applied on the surface of the underlying film and the other processesare implemented in the same manner with the case of the 58-thembodiment, thus obtaining the state shown in FIG. 48E. Beside themethod of using the solution, sputtering, CVD or adsorption may be usedas the method for introducing nickel element

[65-th Embodiment]

A 65-th embodiment relates to a case of improving the crystallinity ofan island pattern formed of a crystal silicon film obtained in theprevious step by irradiating laser light in the state shown in FIG. 49E,the state shown in FIG. 50A or the state shown in FIG. 51A. Apredetermined annealing effect can be obtained with relatively lowirradiation energy density by irradiating the laser light in the stateshown in FIGS. 49E, 50A and 51A. This effect is considered to have beeneffected because the laser energy is irradiated to a spot of small area,thus enhancing the efficiency of energy utilized in the annealing.

[66-th Embodiment]

A 66-th embodiment relates to a case of fabricating a bottom-gate typethin film transistor. FIGS. 53A through 53F shows the process forfabricating the thin film transistor of the present embodiment. Atfirst, a silicon oxide film 517 is formed as an underlying film on aglass substrate 516.

Next, while a gate electrode 518 is formed by using an adequate metallicmaterial or metallic silicide material, aluminum is used here. Afterforming the gate electrode 518, a silicon oxide film 519 which functionsas a gate insulating film is formed. Further, an amorphous silicon film520 is formed by means of plasma CVD. Next, nickel acetate aqueoussolution is applied as shown in FIG. 53B to hold nickel element incontact on the surface of the amorphous silicon film 520 as a liquidfilm 521 of the nickel acetate solution.

Next, a heat treatment is implemented within an nitrogen atmospherecontaining 3 volume % of hydrogen at 650° C. to crystallize theamorphous silicon film 520 and to obtain a crystal silicon film 522. Aheat treatment is implemented to the crystal silicon film within anoxygen atmosphere containing 5 volume % of HCl and 100 ppm (volume) ofNF₃ at 650° C. After obtaining the state shown in FIG. 53C by forming athermal oxide film 523 as the result of the heat treatment, it isremoved.

Next, the crystal silicon film 522 and the silicon oxide film 519 arepatterned to form the gate insulating film 525 and an active layer 526of the thin film transistor. Further, a resist mask 524 is placed asshown in FIG. 53D. In the state shown in FIG. 53D, impurity ions areinjected to form a source and drain regions. Here, P (phosphorus) ionsare injected in order to fabricate a N-channel type thin filmtransistor. In this step, the source region 527 and the drain region 528are formed.

After that, isotropic ashing is implemented to cause the resist mask 524to recede as a whole. That is, as indicated by the reference numeral 529in FIG. 53E, the size of the resist mask 524 is reduced as a whole.Thus, the receded resist mask 529 is obtained. Next, P ions are injectedagain in the state shown in FIG. 53E.

This step is implemented with less dosage than the dosage of P ions inthe step in FIG. 53D. Thus, low concentration impurity regions 530 and531 are formed. Next, metallic electrodes 532 and 533 are formed. Here,the electrode 532 is the source electrode and the electrode 533 is thedrain electrode. Thus, the bottom-gate type thin film transistor iscompleted.

[Mode as Premise of 67-th Embodiment]

FIGS. 54A through 54E as well as FIGS. 55A and 55B are diagrams forexplaining the fabrication processes of a thin film transistor (TFT) orare diagrams for explaining the fabrication processes of a TFT in a67-th embodiment. Then, a concrete mode of the invention which is apremise of the 67-th embodiment will be explained at first based onFIGS. 54 through 55.

As shown in FIG. 54A, an underlying film 535, an amorphous silicon film536 are laminated sequentially on a glass substrate 534 and Ni layer 537is formed on the surface of the amorphous silicon film 536. When a heattreatment is implemented in this state, the amorphous silicon film 536is crystallized and a crystal silicon film 538 is formed as shown inFIG. 54B. Although the step for forming the nickel layer 537 is notessential in the present mode, nickel allows the heating temperature inthe crystallization step to be lowered and the heating time to beshortened because it functions as a catalyst for lowering thermal energyrequired for the crystallization.

While Fe, Co, Ru, Rh, Pd, Os, Ir, Pt, Cu and Au may be used as such acatalytic element, beside nickel (Ni), the catalytic effect of nickel ismost remarkable. It is noted that the crystal silicon film may be formedby using known technologies or the like without using nickel element.Further, laser light may be irradiated as the crystallization stepinstead of the heat treatment. Further, it is possible to implementlight annealing by means of laser light or infrared rays or thermalannealing after forming the crystal silicon film.

FIG. 54C shows a thermal oxidation step. That is, with the formation ofSi—O coupling, non-coupled Si is also generated. This excessive Sidiffuses within the crystal silicon film 538 from the interface betweenthe thermal oxide film 539 and the crystal silicon film 538 and coupleswith dangling bond of Si existing in the grain boundary, thuspassivating the defects at the grain boundary of the crystal siliconfilm 538. Thereby, the mobility of the TFT composed of the crystalsilicon film 538 may be improved.

Further, Si which passivates the defects allows to eliminate thehydrogen plasma treatment because it will not readily desorb from thecrystal silicon film 538 like H in the following fabrication processinvolved in heating. For example, concerning to a N-channel TFTfabricated in accordance to the inventive method for fabricating asemiconductor device, the mobility after the hydrogen plasma treatmentincreases only 10 to 20% as compared to that before the hydrogen plasmatreatment. It indicates that the defects of the crystal silicon film 538has been fully passivated in the thermal oxidation step and that Siwhich passivates the defects is not desorbed during the fabricationstep.

Taking into consideration that the object of the thermal oxidation stepin the present invention is to supply Si for passivating the defects ofthe grain boundary of the crystal silicon film and that the crystalsilicon film 538 composes the active layer of the TFT, the thermal oxidefilm 539 may be formed in a thickness of about 200 to 500 angstromwithout considering the film quality such as pressure resistance.Further, it is intended to fabricate the TFT on the glass substrate inthe present invention, the thermal oxidation step has to be implementedunder the condition in which the distortion or deformation of thesubstrate caused by heat is permissible. For example, the upper limit ofthe heating temperature may be set by the criterion of the distortionpoint of the glass substrate.

In the present invention, the thermal oxidation step is implemented inan oxidizing atmosphere in which fluorine compound is added. Inconcrete, the thermal oxidation is implemented within an atmosphere inwhich NF₃ gas or the like is added to oxygen gas. The thermal oxide filmmay be grown to several hundreds angstrom by heating for several to 10and several hours at temperatures below the distortion point of theglass substrate by adequately controlling the concentration of NF₃ gas.

Although the growth of the thermal oxide film is accelerated by addinggas which supplies Cl radicals like Cl to the oxidizing atmosphere,beside the gas which supplies fluorine radicals like the NF₃ gas, it isnot suitable because it takes time if the thermal oxide film is to beformed in a thickness of several hundreds angstrom by heating intemperatures below the distortion point of the glass substrate, e.g. inthe temperature range of around 600 to 500° C. The thermal oxide filmmay be formed in a thickness of around 200 angstrom by heating for fourhours at 600° C. within the oxidizing atmosphere in which 450 ppm of NF₃gas is added to oxygen gas.

Further, because the fluorine radicals are supplied concentrating to aconvex portion on the surface of the crystal silicon film 538 in thethermal oxidation step, thermal oxidation of the convex portion proceedsmost and that of a concave portion is suppressed. Further, because thethermal oxide film 539 has fluorine in high concentration and stress isrelaxed, the thermal oxide film 539 is formed in a uniform thickness onthe surface of the crystal silicon film 538 such that the convex portionis rounded.

Because the heating temperature and heating time need to be determinedin the thermal oxidation step such that the distortion and deformationof the substrate fall within the permissible range, the concentration offluorine compound within the thermal oxidation atmosphere may increase.As a result, the thermal oxide film 539 may contain a large amount offluorine, thus forming Si—F bond as a result. However, because thethermal oxide film 539 is formed as a film grown to supply Si whichpassivates the defects at the grain boundary of the crystal silicon filmand as a film to be eliminated later, it is not required to have thehigh function and high reliability like the gate insulating film and theinstability like Si—F bond existing within the thermal oxide film 539 orits pressure resistance does not matter.

FIG. 54D shows the active layer 540 and the gate insulating film 541formed by patterning the crystal silicon film 538 after eliminating thethermal oxide film 539. Although it is possible to adopt the thermaloxidation to form the gate insulating film 541, the film quality of thethermal oxide film obtained by the thermal oxidation in a lowtemperature which causes the deformation of the glass substrate in apermissible degree is not good. Due to that, in the present mode, thegate insulating film is formed by deposition such as the plasma CVD orsputtering in order to stably obtain the gate insulating film having thepredetermined characteristics.

Because the surface of the active layer 540 (the crystal silicon film538) is flattened by going through the thermal oxidation step in thepresent mode, the gate insulating film may be formed with a good coatingcapability even if it is formed by the deposition. Thereby, theinterfacial level between the gate insulating film and the active layermay be lowered.

While the crystal silicon film obtained by irradiating laser light hasan excellent crystallinity, a ridge having a sharp projection is formedon the surface thereof. For instance, when laser annealing isimplemented after heating an amorphous silicon film of about 700angstrom to crystallize it, a ridge having about 100 to 300 angstrom ofheight is formed on the surface thereof.

Then, the difference of height on the surface of the crystal siliconfilm may be reduced to about several tens angstrom by forming a thermaloxide film of about 500 angstrom thick by implementing thermal oxidationfor 12 hours within an atmosphere in which about 450 ppm of NF₃ gas isadded to oxygen gas. Accordingly, the insulating film may be depositedby CVD favorably also on the surface of the crystal silicon filmcrystallized by laser light.

FIG. 55A shows a step for doping impurity ions. A gate electrode 542functions as a mask and a source region 544, a drain region 545 and achannel region 546 are formed in a manner of self-alignment. Further, aninterlayer insulating film 547, electrodes 548 and 549 are formed asshown in FIG. 55B, thus completing the TFT.

[67-th Embodiment]

A 67-th embodiment relates to a case of fabricating a TFT using thesilicon film crystallized by utilizing the catalytic action of the metalelement which promotes crystallization of silicon. FIGS. 54 through 55are diagrams for explaining steps for fabricating the TFT according tothe present embodiment, or section views showing each step. Nickel isused as the metal element in the present embodiment.

As shown in FIG. 54A, a silicon oxide film is formed as an underlyingfilm 535 on a glass substrate 534 (Corning 1737, distortion point: 667°C.) by means of plasma CVD or low pressure CVD. Next, a substantiallyintrinsic (I type) amorphous silicon film 536 is formed by the plasmaCVD or low pressure CVD in a thickness of 700 to 1000 angstrom. Here,the low pressure CVD is used for forming the both films. The thicknessof the amorphous silicon film 536 is 700 angstrom.

UV (ultraviolet) rays are irradiated to the surface of the amorphoussilicon film 536 within an oxidizing atmosphere to form an oxide filmnot shown in a thickness of several 20s angstrom. After that, solutioncontaining nickel element is applied on the surface of the oxide film.The oxide film is formed to improve wetting of the surface of theamorphous silicon film 536 to suppress the solution from being repelled.In the present embodiment, nickel acetate aqueous solution containing 55ppm of nickel is used as the solution containing nickel element.

In the present embodiment, the nickel acetate solution is applied by aspinner and is then dried to form a nickel layer 537. Although thenickel layer 537 may not be formed as a complete film, nickel element isheld in contact on the surface of the amorphous silicon film 536 via theoxide film not shown in this state. It is noted that while the solutionin which nickel salt is diluted may be used, preferably a solutioncontaining nickel in concentration of about 1 to 100 ppm may be used.

Here, it is difficult to obtain the effect of promoting thecrystallization if the nickel concentration within the amorphous siliconfilm is less than 1×10¹⁶ atoms/cm⁻³. When the nickel concentration isabove 5×10¹⁹ atoms/cm⁻³, the characteristics as a semiconductor of thesilicon film thus obtained is lost and the characteristic as a metalappears. Therefore, conditions such as the concentration of nickelwithin the nickel acetate solution, a number of times of application andan applied amount are set in advance so that the average nickelconcentration within the silicon film finally obtained falls in therange of 1×10¹⁶ atoms/cm³ to 5×10¹⁹ atoms/cm³. It is noted that theconcentration of nickel may be measured by SIMS (secondary ion massspectrometry).

In the state in which the nickel element is held on the surface of theamorphous silicon film 536 as shown in FIG. 54A, a heat treatment isimplemented within a nitrogen atmosphere to crystallize the amorphoussilicon film 536 and to form a crystal silicon film 538 as shown in FIG.54B. While it is necessary to heat at a temperature more than 450° C. inorder to crystallize silicon, it is desirable to heat at a temperaturemore than 550° C. because it takes several tens hours to crystalize theamorphous silicon film in the temperature range of 450 to 500° C. It isnoted that the heating temperature must be within the range in which thedeformation or contraction of the glass substrate caused by heating ispermissible in all the steps, not only the crystallization step shown inFIG. 54B.

The reference of the upper limit of the heating temperature may be setat the distortion point of the substrate. Because the glass substrate534 whose distortion point is 667° C. is used in the present embodiment,the heat treatment is implemented at 620° C. for four hours. Thereby,nickel heads from the surface of the amorphous silicon film 536 to theinterface with the underlying film 535 and diffuses in a direction atalmost right angles with the face of the glass substrate 534. Then,along that, crystal growth of silicon proceeds, thus forming the crystalsilicon film 538. This crystal growth advances unorderly in a directionvertical to the glass substrate 534. Such growth process will bereferred to as a vertical growth here.

It is noted that the crystallinity of the crystal silicon film 538 maybe improved further by implementing light annealing by means of laserlight, infrared rays or ultraviolet rays, or heat annealing as necessaryafter the crystallization step. The light annealing and the heatannealing may be used in combination. However, when the laser annealingis implemented, the thickness of the amorphous silicon film 536, i.e.the starting film of the crystal silicon film 538, is set below 1000angstrom, or preferably 700 to 800 angstrom, in order to effectivelysupply thermal energy to the crystal silicon film 538.

Next, a thermal oxide film 539 is formed in a thickness of 200 to 500angstrom on the surface of the crystal silicon film 538 by heating in anoxygen atmosphere containing fluorine atoms. In the present embodiment,the thermal oxide film 539 is formed in a thickness of about 200angstrom by heating for four hours at 600° C. within the atmosphere inwhich 400 ppm of NF₃ is added to oxygen gas.

As a result, the thickness of the crystal silicon film 538 which hasbeen about 700 angstrom before the formation of the thermal oxide film539 is reduced to about 600 angstrom. Because the crystal silicon film538 composes the active layer of the TFT in the end, the thickness ofthe amorphous silicon film 536 has to be determined taking the thicknessof the thermal oxide film 539 into consideration so that the activelayer having the required thickness can be obtained.

As the thermal oxide film 539 is formed on the surface of the crystalsilicon film 538, non-coupled Si is also generated. This excessive Sidiffuses within the crystal silicon film 538 from the interface betweenthe thermal oxide film 539 and the crystal silicon film 538 and coupleswith dangling bond of Si existing in the grain boundary, thus reducingthe density of defects at the grain boundary of the crystal silicon film538. Further, because Si which passivates the defects will not readilydesorb from the crystal silicon film 538 like H in the followingfabrication process utilizing heating, the crystal silicon film 538 issuitable as a material of a semiconductor device such as a TFT.

Further, because the oxidation rate of the convex and concave portionsis different, the convex portion on the surface of the crystal siliconfilm 538 is rounded and is flattened. It is noted that because theridges are formed on the surface of the crystal silicon film 538 when itis irradiated by laser light, the conditions of the thermal oxidationprocess such as the thickness of the thermal oxide film 539 and theconcentration of NF₃ gas are set taking the thickness of the crystalsilicon film 538 after the thermal oxidation process into considerationso that the ridges can be flattened or removed as much as possible.

Next, the thermal oxide film 539 is eliminated by etching as shown inFIG. 54D. In etching the film, etching solution or etching gas havinghigh etching rate for silicon oxide and silicon is used. While bufferhydrofluoric acid or other hydrofluoric etchants may be preferably usedas the etchant, the thermal oxide film 539 is eliminated by means of wetetching using the buffer hydrofluoric acid in the present embodiment.

Next, the crystal silicon film 538 is patterned into an island shape toform an active layer 540 of the TFT. Then, a silicon oxide film isformed as a gate insulating film 541 in a thickness of 1000 angstrom bymeans of plasma CVD. Because the surface of the active layer 540 hasbeen flattened in the thermal oxidation process, the gate insulatingfilm 541 may be deposited with a favorable coating capability. Afterthat, an aluminum film containing a trace amount of scandium not shownis formed in a thickness of 6000 angstrom on the surface of the gateinsulating film 541 by means of electron beam deposition and ispatterned as shown in FIG. 54E to form a gate electrode 542.

Then, an oxide layer 543 is formed by implementing anodization withinelectrolyte by setting the gate electrode 542 at the anode. In thiscase, the anodic oxide layer 543 having a dense structure is formed in athickness of 2000 angstrom by using the ethylene glycol solutioncontaining 3 weight % of tartaric acid as the electrolyte, by settingthe gate electrode 542 at the anode and platinum at the cathode and byapplying voltage. The thickness of the anodic oxide 543 may becontrolled by adjusting the application time of the voltage.

Next, as shown in FIG. 55A, impurity ions which give one conductive typeto the active layer 540 are injected by means of ion injection or plasmaion injection in order to form a source region 544 and a drain region545. Here, P (phosphorus) ions are injected to the active layer 540 byusing phosphine diluted to 1 to 10 volume % by H₂ gas in order tofabricate an N-channel type thin film transistor. On the other hand, B(boron) ions are injected when a P-channel type thin film transistor isto be fabricated by using diborane diluted similarly to 1 to 10%. In thepresent embodiment, P and B ions are injected by means of the ioninjection to fabricate the N-channel and P-channel type TFTs,respectively.

When the impurity ions are injected to the active layer 540, the gateelectrode 542 and the anodic oxide 543 around thereof function as amask, thus defining the regions into which the impurity ions have beeninjected as the source region 544 and the drain region 545 and definingthe region into which no impurity ion has been injected as a channel546. It is noted that the doping conditions such as the dosage and theacceleration voltage are controlled so that the concentration of theimpurity ions in the source region 544 and the drain region 545 turnsout to be in the range of 3×10¹⁹ to 1×10²¹ atoms/cm³. Further, after thedoping, laser light is irradiated to activate the impurity ions injectedto the source region 544 and the drain region 545.

Next, a silicon oxide film is formed by mean of plasma CVD in athickness of 7000 angstrom as indicated as an interlayer insulating film547 in FIG. 55B. Then, contact holes are created to form electrodes 548and 549 which are connected with the source region 544 and the drainregion 545, respectively, by materials mainly composed of aluminum.

Finally, a hydrogen plasma treatment is implemented at 300° C., thuscompleting the thin film transistor shown in FIG. 55B. It is noted thatthe main purpose of this hydrogen plasma treatment is not to passivatethe defects of the active layer 540 but to passivate the interfacebetween the active layer 540 and the electrodes 548 and 549 made fromaluminum.

There is no big change in the field effect mobility of the P-channeltype TFT fabricated in accordance to the fabrication process of thepresent embodiment before and after the implementation of the hydrogenplasma treatment. This is construed that it does not mean that thethermal oxidation step shown in FIG. 54C has no passivation effect, butthat it means that the passivation of the defects at the grain boundaryof the active layer 540 is not the best means in improving the fieldeffect mobility in the P-channel type TFT, as it is supposed from thefact that the field effect mobility of the P-channel type TFT is notremarkably improved by the passivation only by way of the hydrogenplasma treatment.

On the other hand, in the N-channel type TFT fabricated in accordance tothe fabrication process of the present embodiment, while the fieldeffect mobility has been 200 cm²·V⁻¹·s⁻¹ before the implementation ofthe hydrogen plasma treatment, the field effect mobility increases onlyby about 10 to 20% after the implementation. This fact indicates thatalthough a N-channel type TFT has not been practical unless it istreated by hydrogen plasma in the past, a practical N-channel type TFTmay be fabricated only by implementing the thermal oxidation by addingNF₃ like the present embodiment.

That is, it indicates that not so many defects at the grain boundary ofthe active layer 540 have been passivated by hydrogen in the hydrogenplasma treatment, but many of the defects at the grain boundary arepassivated in the thermal oxidation step shown in FIG. 54C. Accordingly,the most of the defects passivated by the hydrogen plasma treatment ofthe present embodiment is those generated after the thermal oxidationstep, i.e. mainly those generated in forming the electrodes 548 and 549.Further, the defects at the grain boundary of the active layer 540 havebeen passivated by Si in the present embodiment. Because Si does notdesorb readily from the active layer due to the thermal influence likeH, the present invention allows the TFT having the excellent heatresistance and the high reliability to be formed.

[68-th Embodiment]

A 68-th embodiment relates to a case of fabricating a TFT using thesilicon film crystallized by utilizing the catalytic action of the metalelement which promotes crystallization of silicon. FIGS. 56 through 57are diagrams for explaining steps for fabricating the TFT according tothe present embodiment, or section views showing each step. Nickel isused as the metal element in the present embodiment.

As shown in FIG. 56A, a silicon oxide film is formed as an underlyingfilm 551 on a glass substrate 550 (Corning 1737, distortion point: 667°C.) in a thickness of 3000 angstrom by means of plasma CVD or lowpressure CVD. The plasma CVD is used here. Next, a substantiallyintrinsic amorphous silicon film 552 is formed by the plasma CVD or lowpressure CVD in a thickness of 700 to 1000 angstrom. Here, the thicknessof the amorphous silicon film 552 is set at 1000 angstrom.

UV (ultraviolet) rays are then irradiated to the surface of theamorphous silicon film 552 within an oxidizing atmosphere to form anoxide film not shown in a thickness of several tens angstrom. The oxidefilm is formed to improve wetting of the surface of the amorphoussilicon film 552 to suppress the solution from being repelled. Afterthat, the solution containing nickel element is applied on the surfaceof the oxide film. In the present embodiment, nickel acetate aqueoussolution containing 55 ppm of nickel is used as the solution containingnickel element.

Then, a mask film 553 having an opening 554 formed of a silicon oxidefilm of 1500 angstrom thick is formed. The opening 554 has a shape ofslit in the longitudinal direction in the direction vertical to thedrawing. Preferably, the opening 554 has a width of 20 μm or more.Meanwhile, the size in the longitudinal direction is determinedadequately corresponding to the size of the substrate.

Next, the nickel acetate solution containing 55 ppm of nickel element isapplied by a spinner and is then dried to form a nickel layer 555.Although the nickel layer 555 may not be formed as a complete film,nickel element is held in contact on the surface of the amorphoussilicon film 552 via the oxide film not shown at the opening 554 of themask film 553 in this state. It is noted that while the solution inwhich nickel salt is diluted may be used, preferably a solutioncontaining nickel in concentration of about 1 to 100 ppm may be used.

Then, a heat treatment is implemented for four hours at 620° C. tocrystallize the amorphous silicon film 552 and to form a crystal siliconfilm 556. Because the crystal grows in the amorphous silicon film 552from the surface of the region 557 exposed at the opening 554 of themask film 553 to the underlying film 551, so that the crystal grows inthe region 557 vertically.

On the other hand, in the region 558, the crystal grows in parallel withthe face of the substrate 550 as indicated by arrows in FIG. 56Bstarting from the vertically grown region 557. The crystallizationprocess in which the crystal grows in one direction as such will bereferred to as lateral growth. Accordingly, the region 558 of thecrystal silicon film 556 is the laterally grown region.

After that, the mask film 553 made from the silicon oxide film isremoved. Next, a thermal oxide film 559 is formed in a thickness of 200to 500 angstrom on the surface of the crystal silicon film 556 byheating in an oxygen atmosphere containing fluorine atoms. It is notedthat the crystallinity of the crystal silicon film 556 may be improvedfurther by implementing light annealing by means of laser light orinfrared rays, or heat annealing, as necessary after the crystallizationstep. The light annealing and the heat annealing may be used incombination.

The thermal oxide film 559 is formed in a thickness of about 500angstrom by heating for 12 hours at 600° C. within the atmosphere inwhich 450 ppm of NF₃ is added to oxygen gas. As a result, the thicknessof the crystal silicon film 556 which has been about 1000 angstrombefore the formation of the thermal oxidation is reduced to about 750angstrom.

As the thermal oxide film 559 is formed on the surface of the crystalsilicon film 556, non-coupled Si is also generated. The Si atoms couplewith dangling bond of Si at the grain boundary, thus passivating thedefects of the crystal silicon film 556. The density of the defects atthe grain boundary of the crystal silicon film 556 may be fully reducedby forming the silicon oxide film 559 in a thickness of about 500angstrom with respect to the crystal silicon film 556 having thethickness of 1000 angstrom.

Next, the thermal oxide film 559 is eliminated by etching as shown inFIG. 56D. In etching the film, etching solution or etching gas having ahigh etching rate for silicon oxide and silicon is used. In the presentembodiment, the thermal oxide film 559 is eliminated by means of wetetching using the buffer hydrofluoric acid.

Next, the crystal silicon film 556 is patterned into an island shape toform an active layer 560 of the TFT as shown in FIG. 56E. In this case,preferably, the active layer 560 is composed of only the laterally grownregion 558. Then, a silicon oxide film 561 which composes a gateinsulating film is formed on the surface of the active layer 560 bymeans of plasma CVD or low pressure CVD. The low pressure CVD is usedhere.

Further, an aluminum film which composes a gate electrode 562 is formedon the surface of the silicon oxide film 561 in a thickness of 5000angstrom by sputtering. Containing a small amount of scandium beforehand allows to suppress hillock or whisker from being produced in thelater heating step. 0.2 weight % of scandium is contained here.

Then, after anodizing the surface of the aluminum film to form a verythin and dense anodic oxide not shown, a resist mask 563 is formed onthe surface of the aluminum film. In this case, because the dense anodicoxide not shown is formed on the surface of the aluminum film, the mask563 may be formed in close adhesion. Next, the aluminum film is etchedby using the resist mask 563 to form a gate electrode 562 as shown inFIG. 56E.

Then, the gate electrode 562 is anodized while leaving the resist mask563 as shown in FIG. 57A to form a porous anodic oxide 564 in athickness of 4000 angstrom. At this time, because the resist mask 563 isadhering on the surface of the gate electrode 562, the porous anodicoxide 564 is formed only on the sides of the gate electrode 562.

Next, the gate electrode 562 is anodized again within the electrolyteafter peeling off the resist mask 563 to form a dense anodic oxide 565in a thickness of 1000 angstrom. The anodic oxides 564 and 565 describedabove may be formed differently by clanging the electrolytes to be used.When the porous anodic oxide 564 is to be formed, acidic solutioncontaining 3 to 20 weight % of citric acid, oxalic acid, chromic acid orsulfuric acid may be used. Here, acidic aqueous solution containing 5weight % of oxalic acid is used.

When the dense anodic oxide 565 is to be formed on the other hand,electrolyte in which ethylene glycol solution containing 3 to 10 weight% of tartaric acid, boric acid or nitric acid is prepared so as to havepH of about 7 may be used. Here, the ethylene glycol solution containing5 weight % of tartaric acid and which is prepared so as to have pH=7 isused.

Next, as shown in FIG. 57C, the silicon oxide film 561 is etched byusing the gate electrode 562 and the porous anodic oxide 564 around itand the dense anodic oxide 565 as a mask to form a gate insulating film566. Next, as shown in FIG. 57D, impurities which give a conductive typeto the active layer 560 are injected by using the gate electrode 562,the dense anodic oxide 565 and the gate insulating film 566 as a mask bymeans of ion doping after eliminating the porous anodic oxide 564.

In the present embodiment, P (phosphorus) ions are injected by usingphosphine as the doping gas to fabricate the N-channel TFT. It is notedthat the doping conditions such as the dosage and the accelerationvoltage are controlled so that the gate insulating film 564 functions asa semi-permeable mask during the doping. As a result of theabove-mentioned doping, phosphorus ions are injected in highconcentration to the region not covered by the gate insulating film 566,thus forming a source region 567 and a drain region 568.

Further, P ions are injected in low concentration to the region coveredonly by the gate insulating film 566, thus forming low concentrationimpurity regions 569 and 570. Because no impurity is injected to theregion right below the gate electrode 562, a channel 571 is formed.After the doping, thermal annealing, laser annealing or the like isimplemented to activate the doped P ions. Here, the thermal annealing isapplied.

Because the low concentration impurity regions 569 and 570 function ashigh resistant regions, they contribute in the reduction of OFF current.The low concentration impurity region 570 is called specifically as theLDD. Further, because an offset structure in which the impurity regionis shifted from the end face of the gate electrode 562 may be allowed,the OFF current may be reduced further.

Next, a silicon oxide film is formed by mean of plasma CVD in athickness of 5000 angstrom as indicated as an interlayer insulating film572 as shown in FIG. 57E. It is noted that beside the mono-layer of thesilicon oxide film, a mono-layer of a silicon nitride film or alaminated film of the silicon oxide film and the silicon nitride filmmay be formed as the interlayer insulator 572.

Next, the interlayer insulator 572 made from the silicon oxide film isetched by etching and contact holes are created respectively at thesource region 567 and the drain region 568. Then, an aluminum film isformed in a thickness of 4000 angstrom by means of sputtering and ispatterned to form electrodes 573 and 574 at the contact holes at thesource region 567 and the drain region 568.

Finally, a heat treatment is implemented at 300° C. in a hydrogenatmosphere. It is noted that the main purpose of this hydrogen plasmatreatment is not to passivate the defects of the active layer 560 but topassivate the interface between the active layer 560 and the electrodes573 and 574 made from aluminum. Through the above process, the thin filmtransistor having the LDD structure is fabricated.

In the N-channel type TFT fabricated in accordance to the fabricationprocess of the present embodiment, the field effect mobility after theimplementation of the hydrogen plasma treatment increases only by about10 to 20% as compared to that before the implementation of the hydrogenplasma treatment. This fact indicates that although a N-channel type TFThas not been practical unless it is treated by hydrogen plasma in thepast, the defects at the grain boundary of the active layer 560 has beeneffectively passivated by only implementing the thermal oxidation byadding NF₃ like the step shown in FIG. 56C.

[69-th Embodiment]

A 69-th embodiment relates to a case of fabricating a CMOS type TFT inwhich a N-channel type TFT and a P-channel type TFT are combined in acomplementary manner. FIGS. 58 through 59 are diagrams for explainingsteps for fabricating the TFT according to the present embodiment.

As shown in FIG. 58A, a silicon oxide film is formed as an underlyingfilm 576 on a glass substrate 575 (Corning 1737) in a thickness of 2000angstrom. Next, a substantially intrinsic amorphous silicon film isformed by the plasma CVD or low pressure CVD in a thickness of 700 to1000 angstrom. Then, a crystal silicon film 577 is formed by the methodshown in the 67-th embodiment. It is noted that the amorphous siliconfilm may be crystallized by using adequate methods such as heattreatment or irradiation of laser. The process which follows is the sameeven in those cases.

A thermal oxide film 578 is formed in a thickness of 200 angstrom asshown in FIG. 58B by implementing thermal oxidation for 2 hours withinan oxygen atmosphere containing 400 ppm of NF₃ at 600° C. to passivatethe defects at the grain boundary of the crystal silicon film 577 by Si.As a result, the crystal silicon film 577 turns out to be a suitablesemiconductor material for TFTs and the like.

Next, after eliminating the thermal oxide film 578 by using etchant madefrom buffer hydrofluoric acid, the crystal silicon film 577 is patternedinto a shape of island to form active layers 579 and 580, respectively.Further, a silicon oxide film 581 which composes a gate insulating filmis deposited in a thickness of 1500 angstrom by means of plasma CVD. Itis noted that the active layer 579 composes the N-channel type TFT andthe active layer 580 composes the P-channel type TFT.

Then, an aluminum film which composes gate electrodes 582 and 583 isformed in a thickness of 4000 angstrom by sputtering. 0.2 weight % ofscandium is contained in the aluminum film before hand to suppresshillock or whisker from being produced. Then, the aluminum film isanodized within the electrolyte to form a dense anodic oxide of about100 angstrom on the surface of the film. Then, a photoresist mask 585 isformed on the surface of the anodic oxide film to pattern the aluminumfilm and to form the gate electrodes 582 and 583, respectively.

Further, while attaching the photoresist mask 585, the gate electrodes582 and 583 are anodized again to form anodic oxides 586 and 587. As theelectrolyte, acidic solution containing 3 to 20 weight % of citric acid,oxalic acid, chromic acid or sulfuric acid may be used. Aqueous solutioncontaining 4 weight % of oxalic acid is used in the present embodiment.

In the state in which the photoresist mask 585 and the anodic oxide film584 exist on the surface of the gate electrodes 582 and 583, porousanodic oxides 586 and 587 are formed only on the sides of the gateelectrodes 582 and 583. The range of growth of the porous anodic oxides586 and 587 may be controlled by adjusting the treatment time of theanodization. This range of growth determines the length of the lowconcentration impurity region (LDD region). In the present embodiment,the porous anodic oxides 586 and 587 are grown in length of 7000angstrom.

Next, after removing the photoresist mask 585, the gate electrodes 582and 583 are anodized again to form dense and rigid anodic oxide films588 and 589. In the present embodiment, ethylene glycol solutioncontaining 3 weight % of tartaric acid is neutralized by aqueous ammoniato pH 6.9 to use as its electrolyte.

Next, P (phosphorus) ions are injected to the island active layers 579and 580 by means of ion doping by using the gate electrodes 582 and 583as well as the porous anodic oxides 586 and 587 as a mask. As dopinggas, phosphine which has been diluted by hydrogen to 1 to 10 volume % isused. While the doping is implemented with acceleration voltage in therange of 60 to 90 kV and the dosage in the range of 1×10¹⁴ to 8×10¹⁵atoms/cm², the acceleration voltage is set at 80 kV and the dosage isset at 1×10¹⁵ atoms/cm² in the present embodiment.

At this time, although P (phosphorus) ions do not transmit through theporous anodic oxides 586 and 587, they transmit through the gateinsulating film 581 and are injected to island silicon 579 and 580. As aresult, N-type impurity regions 590 through 593 are formed respectivelyas shown in FIG. 59E.

Next, after eliminating the dense anodic oxide film 584 by bufferhydrofluoric acid, the porous anodic oxides 586 and 587 are eliminatedby mixed acid in which phosphoric acid, acetic acid and nitric acid aremixed as shown in FIGS. 59E and 59F. Because the porous anodic oxides586 and 587 may be readily eliminated, the dense and rigid anodic oxides588 and 589 will not be etched.

Next, P (phosphorus) ions are doped again. The doping is implementedwith acceleration voltage in the range of 60 to 90 kV and the dosage inthe range of 1×10¹² to 1×10¹⁴ atoms/cm². The acceleration voltage is setat 80 kV and the dosage at 1×10¹⁴ atoms/cm² in the present embodiment.At this time, although P (phosphorus) ions do not transmit through thegate electrodes 582 and 583, they transmit through the gate insulatingfilm 581 and are injected to the active layers 579 and 580. Accordingly,the region into which phosphorus ions have been injected twice turn outto be N-type high concentration impurity regions 594 through 597 and theregions into which phosphorus ions have been injected once turn out tobe N-type low concentration impurity regions 598 through 601.

Then, as shown in FIG. 59G, while the region which turns out to be theN-channel TFT is covered by polyimide or heat resistant resist 602, thepolyimide is used here. After that, boron ions are doped in order toinvert the conductive type of the active layer 580 from the N-type tothe P-type. Diborane which has been diluted to about 1 to 10 volume % byhydrogen is used as the doping gas and the acceleration voltage is setat 80 kV and the dosage of boron is set at 2×10¹⁵ atoms/cm².

Because no boron is injected to the region covered by the polyimide 602,it remains to be the N type. Accordingly, the high concentrationimpurity regions 594 and 595 correspond to the source and drain regionsof the N-channel type TFT, respectively, in the active layer 579 andbecause no phosphorus nor boron ion is injected to the region 603 rightbelow the gate electrode 582, it remains to be intrinsic and correspondsto the channel of the TFT.

Because a large amount of boron is injected in the doping of boron ions,no low concentration impurity region (LDD region) is formed and only theP-type high concentration impurity regions 604 and 605 are formed. Thehigh concentration impurity regions 604 and 605 correspond to the sourceand drain regions of the P-channel type TFT, respectively. Further,because no phosphorus and boron ion is injected to the region 606 rightbelow the gate electrode 583, it remains to be intrinsic and correspondsto the channel.

Next, the resist 602 is removed and a silicon oxide film of 1 μm thickis formed as an interlayer insulating film 607 by means of plasma CVD asshown in FIG. 59H. Contact holes are then formed and electrodes, wirings608 to 610 for the source and drain regions are formed by multi-layeredfilm of titanium and aluminum to the contact holes. Finally, a heattreatment is implemented for 2 hours within a hydrogen atmosphere at350° C. The CMOS thin film transistor is completed going through suchprocess.

Because the CMOS structure in which the N-type TFT and the P-type TFTare combined complementarily is formed in the present embodiment,electric power may be lowered in driving the TFTs. Further, because itis arranged so as to dispose the low concentration impurity region 599between the channel 603 and the drain domain 595 of the N-channel typeTFT, it allows to prevent a high electric field from being generatedbetween the channel 603 and the drain 595.

It is noted that the conditions of the thermal oxidation step in whichNF₃ is added are not confined to what have been described in theabove-mentioned embodiments 67 through 69. The concentration of NF₃within the oxygen atmosphere and the like may be determined such thatthe thermal oxide film grows in a thickness of several hundreds angstromby heating for several hours at temperatures below the distortion pointof the glass substrate so that the distortion or deformation of thesubstrate, on which the TFT is formed, caused by the thermal oxidationstep falls within the permissible range. Further, such step may beimplemented in the condition of higher temperature when a high heatresistant substrate such as a quartz substrate is used.

Further, because the Corning 1737 glass whose distortion point is 667°C. is used as the glass substrate in the embodiments 67 through 69, theheating temperature in the thermal oxidation step is set at 600° C.However, if a glass whose distortion point is 593° C. is used forexample, the heating temperature in the thermal oxidation step may bepreferably around 500 to 550° C.

[Examples of Industrial Applicability]

The inventive semiconductor device may be used as a display or the likeof various electric equipments. FIGS. 60 through 61 illustrate some ofthem. FIG. 60A shows a portable information terminal unit, FIG. 61Bshows a HMD (Head Mounting Display) used for an endoscope or fortraining in driver's school, FIG. 60C shows a car navigator, FIG. 61Dshows a portable telephone, FIG. 61E shows a video camera, and FIG. 61Fshows a projector. The uses of the inventive semiconductor device arenot confined to those described above.

As described above, according to the present invention, the metalelement existing within the crystal silicon film obtained by utilizingthe metal element which promote crystallization of silicon may beeliminated or its concentration may be reduced. Further, the utilizationof it allows a thin film semiconductor device having the higherreliability and excellent performance to be obtained.

Further, in the method for fabricating a semiconductor device accordingto the present invention, the thermal oxide film may be grown up toseveral hundreds angstrom of thickness by heating for several hours toten and several hours at a temperature below the distortion point of theglass substrate by arranging so as to grow the thermal oxide film withinan oxidizing atmosphere containing fluorine compound. Still more,because extra Si is generated by growing the thermal oxide film and thedefects at the grain boundary of the crystal silicon film may bepassivated by Si, the hydrogen plasma treatment may be eliminated.

Further, because the surface of the crystal silicon film may beflattened by the thermal oxidation process and because the gateinsulating film composed of a deposited film may be formed with afavorable coating capability even if a step of obtaining the crystalsilicon film through the irradiation of laser light is adopted, theinterfacial level between the gate insulating film and the active layermay be lowered. Because the crystal silicon film is irradiated by laserlight and excels in the crystallinity, the mobility of the semiconductordevice may be improved.

Accordingly, an insulated gate type semiconductor device such as a TFThaving the high mobility and high reliability may be fabricated on thesubstrate, such as a glass substrate, on which it is difficult toimplement a treatment in high temperatures such as around 1000° C. forexample.

While preferred embodiments have been described, variations thereto willoccur to those skilled in the art within the scope of the presentinventive concepts which are delineated by the following claims.

TABLE 1 N-CHANNEL TYPE TFT Ion_1[uA] Ion_2[uA] Ioff_1[pA] Ioff_2[pA]Vth[V] S-value μFE[cm2/Vs] μFE[cm2/Vs] MEASURED (VD = 1 V) (VD = 5 V)(VD = 1 V) (VD = 5 V) Ion/ Ion/ (VD = [mV/dec] (VD = 1 V) (VD = 1 V)POINTS (VG = 5 V) (VG = 5 V) (VG = −6 V) (VG = −1 V) Ioff_1 Ioff_2 5 V)(VD = 1 V) (VG = 5 V) (max)  1 82.474 258.800 0.250 0.700 8.518 8.568−0.40 81.03 157.9 259.7  2 115.200 346.710 0.250 0.750 8.664 8.665 −0.4783.46 213.2 367.3  3 87.440 283.150 0.400 2.600 8.340 8.037 −0.52 88.21159.6 272.4  4 92.343 288.860 3.400 4.950 7.434 7.766 −0.43 96.64 186.7280.8  5 87.488 275.050 0.250 0.900 8.544 9.485 −0.40 82.73 175.1 275.2 6 89.910 275.510 0.200 0.500 8.653 8.741 −0.37 77.36 174.2 286.3  773.921 240.100 0.500 1.200 8.170 8.301 −0.49 84.32 146.0 222.6  8 89.153281.300 0.400 1.300 8.348 8.335 −0.50 80.93 163.4 280.8  9 91.303295.450 0.300 3.000 8.483 7.993 −0.59 75.32 158.9 284.9 10 119.650353.200 0.300 0.650 8.601 8.735 −0.33 74.29 225.9 387.6 11 106.000338.300 0.200 202.950 8.724 6.222 −0.68 78.04 173.1 307.6 12 88.304281.940 0.300 0.950 8.469 8.472 −0.47 85.94 154.8 262.5 13 125.000406.760 0.250 119900.000 8.699 3.531 −0.96 72.53 169.3 383.5 14 82.169268.950 0.150 326.000 8.739 5.916 −0.67 80.80 143.6 227.5 15 92.950311.850 0.300 1895.500 8.491 5.216 −0.79 77.16 142.9 268.7 16 101.370320.300 0.250 2.550 8.608 8.099 −0.58 74.04 161.6 307.3 17 80.820262.500 0.250 6.000 8.510 7.641 −0.57 80.38 141.3 235.1 18 104.650339.110 0.350 67.199 8.476 6.703 −0.61 90.97 175.5 309.0 19 94.875305.850 0.300 0.750 8.500 8.610 −0.37 72.99 167.5 290.2 20 72.710236.550 0.250 1.000 8.464 8.374 −0.39 93.90 134.8 213.7

TABLE 2 P-CHANNEL TYPE TFT Ion_1[uA] Ion_2[uA] Ioff_1[pA] Ioff_2[pA]Vth[V] S-value μFE[cm2/Vs] μFE[cm2/Vs] MEASURED (VD = −1 V) (VD = −5 V)(VD = −1 V) (VD = −5 V) Ion/ Ion/ (VD = [mV/dec] (VD = −1 V) (VD = −1 V)POINTS (VG = −5 V) (VG = −5 V) (VG = 6 V) (VG = 1 V) Ioff_1 Ioff_2 5 V)(VD = −1 V) (VG = −5 V) (max)  1 33.624 68.959 19.300 114.950 6.2415.778 −1.54 101.72 153.1 153.3  2 35.515 73.284 1.850 48.750 7.283 6.177−1.47 80.49 156.4 157.8  3 36.074 77.680 1.300 28.250 7.443 6.439 −1.3678.24 145.9 152.6  4 36.575 82.783 1.850 19.800 7.296 6.621 −1.20 72.41134.6 149.0  5 33.969 69.093 3.550 27.700 6.981 6.397 −1.58 93.42 159.6160.2  6 30.304 63.490 1.750 59.149 7.238 6.031 −1.43 126.33 130.4 133.5 7 34.084 72.148 1.800 40.300 7.277 6.253 −1.44 91.27 147.5 149.0  835.519 76.968 1.450 33.150 7.389 6.366 −1.42 86.21 149.7 153.6  9 31.97065.543 1.450 22.100 7.343 6.472 −1.59 85.42 143.4 146.7 10 38.149 82.8131.000 20.050 7.581 6.616 −1.33 102.60 153.0 159.7 11 36.690 82.061 1.45040.300 7.403 6.309 −1.27 68.85 133.8 141.8 12 32.414 67.774 1.300 33.5507.397 6.305 −1.48 98.81 133.3 135.7 13 40.974 89.958 1.350 20.900 7.4826.634 −1.29 77.82 144.9 162.4 14 36.970 76.551 1.150 27.150 7.507 6.450−1.48 102.67 154.6 155.0 15 37.035 76.234 1.150 16.050 7.508 6.677 −1.4595.96 147.7 156.9 16 29.054 60.005 1.250 32.200 7.366 6.270 −1.60 76.28126.1 127.1 17 34.820 74.960 2.850 18.900 7.087 6.598 −1.35 78.89 135.0140.6 18 41.619 87.679 2.600 44.199 7.204 6.297 −1.37 111.21 162.6 166.519 41.740 85.656 1.150 24.649 7.560 6.541 −1.51 70.01 166.0 175.7 2038.784 82.258 1.400 21.050 7.443 6.592 −1.37 95.85 144.5 157.4

1. A method for fabricating a semiconductor device, comprising: formingan amorphous semiconductor film comprising silicon; providing saidamorphous semiconductor film with a metal element which promotescrystallization of silicon; obtaining a crystal semiconductor filmcomprising silicon by crystallizing said amorphous semiconductor film bya first heat treatment; forming an active layer of a thin filmtransistor by patterning said crystal semiconductor film; forming athermal oxide film over said active layer after the formation of saidactive layer by the patterning, by implementing a second heat treatmentin the temperature range of 500° C. to 700° C. within an atmospherecontaining oxygen, hydrogen, fluorine and chlorine to reduce said metalelement existing within said active layer; and eliminating said thermaloxide film by hydrofluoric acid.
 2. The method of claim 1, wherein thecrystal within said crystal silicon film which has been crystallizedfrom said amorphous silicon film is what crystal lattices liecontinuously in a row.
 3. The method of claim 1, wherein the crystalwithin said crystal silicon film which has been crystallized from saidamorphous silicon film is thin cylindrical crystal or thin flatcylindrical crystal.
 4. The method of claim 1, wherein the crystalwithin said crystal silicon film which has been crystallized from saidamorphous silicon film is a plurality of thin cylindrical crystals orthin flat cylindrical crystals which have grown in parallel or almost inparallel leaving a space therebetween.
 5. The method of claim 1, whereinthe concentration of said metal element within said oxide film is higherthan that of said metal element within said crystal silicon film.
 6. Themethod of claim 1, wherein hydrogen is contained within the atmospherefor implementing the second heat treatment in concentration of more than1% and less than explosion limit.
 7. The method of claim 1, wherein saidfirst heat treatment is carried out in a reducing atmosphere.
 8. Themethod of claim 1, wherein one or a plurality of elements selected fromFe, Ca, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu and Au is used as the metalelement which promotes the crystallization of silicon.
 9. The method ofclaim 1, wherein Ni is used as the metal element which promotes thecrystallization of silicon.
 10. The method of claim 1, wherein laserlight or intense light is irradiated to said crystal silicon film afterobtaining said crystal silicon film by crystallizing said amorphoussilicon film by means of the first heat treatment.
 11. A method forfabricating a semiconductor device, comprising: forming an amorphoussemiconductor film comprising silicon; providing said amorphoussemiconductor film with a metal element which promotes crystallizationof silicon; obtaining a crystal semiconductor film comprising silicon bycrystallizing said amorphous semiconductor film by a first heattreatment to obtain a crystal semiconductor film comprising silicon;forming an active layer of a thin film transistor by patterning saidcrystal semiconductor film; forming a thermal oxide film over saidactive layer after the formation of said active layer by the patterning,by implementing a second heat treatment in the temperature range of 500°C. to 700° C. within an atmosphere containing oxygen, hydrogen, fluorineand chlorine to reduce said metal element existing within said activelayer; and eliminating said thermal oxide film.
 12. The method of claim11 wherein the crystal within said crystal silicon film which has beencrystallized from said amorphous silicon film is what crystal latticeslie continuously in a row.
 13. The method of claim 11, wherein thecrystal within said crystal silicon film which has been crystallizedfrom said amorphous silicon film is thin cylindrical crystal or thinflat cylindrical crystal.
 14. The method of claim 11, wherein thecrystal within said crystal silicon film which has been crystallizedfrom said amorphous silicon film is a plurality of thin cylindricalcrystals or thin flat cylindrical crystals which have grown in parallelor almost in parallel leaving a space therebetween.
 15. The method ofclaim 11, wherein the concentration of said metal element within saidoxide film is higher than that of said metal element within said crystalsilicon film.
 16. The method of claim 12, wherein hydrogen is containedwithin the atmosphere for implementing the second heat treatment inconcentration of more than 1% and less than explosion limit.
 17. Themethod of claim 11, wherein said first heat treatment is carried out ina reducing atmosphere.
 18. The method of claim 11, wherein one or aplurality of elements selected from Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt,Cu and Au is used as the metal element which promotes thecrystallization of silicon.
 19. The method of claim 11, wherein Ni isused as the metal element which promotes the crystallization of silicon.20. The method of claim 11, wherein laser light or intense light isirradiated to said crystal silicon film after obtaining said crystalsilicon film by crystallizing said amorphous silicon film by means ofthe first heat treatment.
 21. A method for fabricating a semiconductordevice, comprising: forming an amorphous semiconductor film comprisingsilicon; providing said amorphous semiconductor film with a metalelement which promotes crystallization of silicon; obtaining a crystalsemiconductor film comprising silicon by crystallizing said amorphoussemiconductor film by a heat treatment; irradiating a laser light tosaid crystal semiconductor film; forming an active layer of a thin filmtransistor by patterning said crystal semiconductor film; forming athermal oxide film over said active layer after the formation of saidactive layer by the patterning, by implementing a second heat treatmentin the temperature range of 500° C. to 700° C. within an atmospherecontaining oxygen, hydrogen, fluorine and chlorine to reduce said metalelement existing within said active layer; and eliminating said thermaloxide film by hydrofluoric acid.
 22. The method of claim 21, wherein thecrystal within said crystal semiconductor film which has beencrystallized from said amorphous semiconductor film is what crystallattices lie continuously in a row.
 23. The method of claim 21, whereinthe crystal within said crystal semiconductor film which has beencrystallized from said amorphous semiconductor film is thin cylindricalcrystal or thin flat cylindrical crystal.
 24. The method of claim 21,wherein the crystal within said crystal semiconductor film which hasbeen crystallized from said amorphous semiconductor film is a pluralityof thin cylindrical crystals or thin flat cylindrical crystals whichhave grown in parallel or almost in parallel leaving a spacetherebetween.
 25. The method of claim 21, wherein the concentration ofsaid metal element within said thermal oxide film is higher than that ofsaid metal element within said crystal semiconductor film.
 26. Themethod of claim 21, wherein hydrogen is contained within the atmospherefor implementing the second heat treatment in concentration of more than1% and less than explosion limit.
 27. The method of claim 21, whereinone or a plurality of elements selected from Fe, Co, Ni, Ru, Rh, Pd, Os,Ir, Pt, Cu and Au is used as the metal element which promotes thecrystallization of silicon.
 28. The method of claim 21, wherein Ni isused as the metal element which promotes the crystallization of silicon.29. A method for fabricating a semiconductor device, comprising: formingan amorphous semiconductor film comprising silicon; providing saidamorphous semiconductor film with a metal element which promotescrystallization of silicon; obtaining a crystal semiconductor filmcomprising silicon by crystallizing said amorphous semiconductor film bya first heat treatment; irradiating a laser light to said crystalsemiconductor film; forming an active layer of a thin film transistor bypatterning said crystal semiconductor film; forming a thermal oxide filmover said active layer after the formation of said active layer by thepatterning, by implementing a second heat treatment in the temperaturerange of 500° C. to 700° C. within an atmosphere containing oxygen,hydrogen, fluorine and chlorine to reduce said metal element existingwithin said active layer; and eliminating said thermal oxide film. 30.The method of claim 29, wherein the crystal within said crystalsemiconductor film which has been crystallized from said amorphoussemiconductor film is what crystal lattices lie continuously in a row.31. The method of claim 29, wherein the crystal within said crystalsemiconductor film which has been crystallized from said amorphoussemiconductor film is thin cylindrical crystal or thin flat cylindricalcrystal.
 32. The method of claim 29, wherein the crystal within saidcrystal semiconductor film which has been crystallized from saidamorphous semiconductor film is a plurality of thin cylindrical crystalsor thin flat cylindrical crystals which have grown in parallel or almostin parallel leaving a space therebetween.
 33. The method of claim 29,wherein the concentration of said metal element within said thermaloxide film is higher than that of said metal element within said crystalsemiconductor film.
 34. The method of claim 29, wherein the hydrogen iscontained within the atmosphere for implementing the second heattreatment in concentration of more than 1% and less than explosionlimit.
 35. The method of claim 29, wherein one or a plurality ofelements selected from Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu and Au isused as the metal element which promotes the crystallization of silicon.36. The method of claim 29, wherein Ni is used as the metal elementwhich promotes the crystallization of silicon.
 37. A method forfabricating a semiconductor device, comprising: forming an amorphoussemiconductor film comprising silicon; providing said amorphoussemiconductor film with a metal element which promotes crystallizationof silicon; obtaining a crystal semiconductor film comprising silicon bycrystallizing said amorphous semiconductor film by a first heattreatment in a reducing atmosphere; irradiating a laser light to saidcrystal semiconductor film; forming an active layer of a thin filmtransistor by patterning said crystal semiconductor film; forming athermal oxide film over said active layer after the formation of saidactive layer by the patterning, by implementing a second heat treatmentin the temperature range of 500° C. to 700° C. within an atmospherecontaining oxygen, hydrogen, fluorine and chlorine to reduce said metalexisting within said active layer; and eliminating said thermal oxidefilm by hydrofluoric acid.
 38. The method of claim 37, wherein thecrystal within said crystal semiconductor film which has beencrystallized from said amorphous semiconductor film is what crystallattices lie continuously in a row.
 39. The method of claim 37, whereinthe crystal within said crystal semiconductor film which has beencrystallized from said amorphous semiconductor film is thin cylindricalcrystal or thin flat cylindrical crystal.
 40. The method of claim 37,wherein the crystal within said crystal semiconductor film which hasbeen crystallized from said amorphous semiconductor film is a pluralityof thin cylindrical crystals or thin flat cylindrical crystals whichhave grown in parallel or almost in parallel leaving a spacetherebetween.
 41. The method of claim 37, wherein the concentration ofsaid metal element within said thermal oxide film is higher than that ofsaid metal element within said crystal semiconductor film.
 42. Themethod of claim 37, wherein hydrogen is contained within the atmospherefor implementing the second heat treatment in concentration of more than1% and less than explosion limit.
 43. The method of claim 37, whereinone or a plurality of elements selected from Fe, Co, Ni, Ru, Rh, Pd, Os,Ir, Pt, Cu and Au is used as the metal element which promotes thecrystallization of silicon.
 44. The method of claim 37, wherein Ni isused as the metal element which promotes the crystallization of silicon.45. A metod for fabricating a semiconductor device, comprising: formingan amorphous semiconductor film comprising silicon; providing saidamorphous semiconductor film with a metal element which promotescrystallization of silicon; obtaining a crystal semiconductor filmcomprising silicon by crystallizing said amorphous semiconductor film bya first heat treatment in a reducing atmosphere to obtain a crystalsemiconductor film comprising silicon; irradiating a laser light to saidcrystal semiconductor film; forming an active layer of a thin filmtransistor by patterning said crystal semiconductor film; forming athermal oxide film over said active layer after the formation of saidactive layer by the patterning, by implementing a second heat in thetemperature range of 500° C. to 700° C. within an atmosphere containingoxygen, hydrogen, fluorine and chlorine to reduce said metal elementexisting within said active layer; and eliminating said thermal oxidefilm.
 46. The method of claim 45, wherein the crystal within saidcrystal semiconductor film which has been crystallized from saidamorphous semiconductor film is what crystal lattices lie continuouslyin a row.
 47. The method of claim 45, wherein the crystal within saidcrystal semiconductor film which has been crystallized from saidamorphous semiconductor film is thin cylindrical crystal or thin flatcylindrical crystal.
 48. The method of claim 45, wherein the crystalwithin said crystal semiconductor film which has been crystallized fromsaid amorphous semiconductor film is a plurality of thin cylindricalcrystals or thin flat cylindrical crystals which have grown in parallelor almost in parallel leaving a space therebetween.
 49. The method ofclaim 45, wherein the concentration of said metal element within saidthermal oxide film is higher than that of said metal element within saidcrystal semiconductor film.
 50. The method of claim 45, wherein thehydrogen is contained within the atmosphere for implementing the secondheat treatment in concentration of more than 1% and less than explosionlimit.
 51. The method of claim 45, wherein one or a plurality ofelements selected from Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu and Au isused as the metal elements which promotes the crystallization ofsilicon.
 52. The method of claim 45, wherein Ni is used as the metalelement which promotes the crystallization of silicon.